Advanced Bash−Scripting Guide
An in−depth exploration of the art of shell scripting Mendel Cooper
2.7 18 April 2004 Revision History Revision 2.5 15 February 2004 'STARFRUIT' release: Bugfixes and more material. Revision 2.6 15 March 2004 'SALAL' release: Minor update. Revision 2.7 18 April 2004 'MULBERRY' release: Minor update.
Revised by: mc Revised by: mc Revised by: mc
This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little snippets of Unix® wisdom and lore. It serves as a textbook, a manual for self−study, and a reference and source of knowledge on shell scripting techniques. The exercises and heavily−commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts. This book is suitable for classroom use as a general introduction to programming concepts. The latest update of this document, as an archived, bzip2−ed "tarball" including both the SGML source and rendered HTML, may be downloaded from the author's home site. See the change log for a revision history.
Dedication For Anita, the source of all the magic
Advanced Bash−Scripting Guide
Table of Contents Chapter 1. Why Shell Programming?...............................................................................................................1 Chapter 2. Starting Off With a Sha−Bang.......................................................................................................3 2.1. Invoking the script............................................................................................................................6 2.2. Preliminary Exercises.......................................................................................................................6 Part 2. Basics.......................................................................................................................................................7 Chapter 3. Special Characters...........................................................................................................................8 Chapter 4. Introduction to Variables and Parameters..................................................................................25 4.1. Variable Substitution......................................................................................................................25 4.2. Variable Assignment.......................................................................................................................27 4.3. Bash Variables Are Untyped..........................................................................................................28 4.4. Special Variable Types...................................................................................................................30 Chapter 5. Quoting...........................................................................................................................................34 Chapter 6. Exit and Exit Status.......................................................................................................................40 Chapter 7. Tests................................................................................................................................................42 7.1. Test Constructs...............................................................................................................................42 7.2. File test operators............................................................................................................................48 7.3. Other Comparison Operators..........................................................................................................51 7.4. Nested if/then Condition Tests.......................................................................................................56 7.5. Testing Your Knowledge of Tests..................................................................................................56 Chapter 8. Operations and Related Topics....................................................................................................58 8.1. Operators.........................................................................................................................................58 8.2. Numerical Constants.......................................................................................................................64 Part 3. Beyond the Basics.................................................................................................................................66 Chapter 9. Variables Revisited........................................................................................................................67 9.1. Internal Variables............................................................................................................................67 9.2. Manipulating Strings.......................................................................................................................83 9.2.1. Manipulating strings using awk............................................................................................87 9.2.2. Further Discussion.................................................................................................................88 9.3. Parameter Substitution....................................................................................................................88 9.4. Typing variables: declare or typeset...............................................................................................97 9.5. Indirect References to Variables.....................................................................................................98 9.6. $RANDOM: generate random integer..........................................................................................100 9.7. The Double Parentheses Construct...............................................................................................109 Chapter 10. Loops and Branches..................................................................................................................111 10.1. Loops..........................................................................................................................................111 10.2. Nested Loops..............................................................................................................................122 10.3. Loop Control...............................................................................................................................122 i
Advanced Bash−Scripting Guide
Table of Contents Chapter 10. Loops and Branches 10.4. Testing and Branching................................................................................................................126 Chapter 11. Internal Commands and Builtins.............................................................................................133 11.1. Job Control Commands..............................................................................................................155 Chapter 12. External Filters, Programs and Commands...........................................................................159 12.1. Basic Commands........................................................................................................................159 12.2. Complex Commands...................................................................................................................163 12.3. Time / Date Commands..............................................................................................................172 12.4. Text Processing Commands........................................................................................................175 12.5. File and Archiving Commands...................................................................................................192 12.6. Communications Commands......................................................................................................206 12.7. Terminal Control Commands.....................................................................................................211 12.8. Math Commands.........................................................................................................................212 12.9. Miscellaneous Commands..........................................................................................................221 Chapter 13. System and Administrative Commands..................................................................................231 Chapter 14. Command Substitution.............................................................................................................255 Chapter 15. Arithmetic Expansion................................................................................................................260 Chapter 16. I/O Redirection...........................................................................................................................261 16.1. Using exec...................................................................................................................................263 16.2. Redirecting Code Blocks............................................................................................................266 16.3. Applications................................................................................................................................270 Chapter 17. Here Documents.........................................................................................................................272 Chapter 18. Recess Time................................................................................................................................281 Part 4. Advanced Topics.................................................................................................................................282 Chapter 19. Regular Expressions..................................................................................................................283 19.1. A Brief Introduction to Regular Expressions..............................................................................283 19.2. Globbing.....................................................................................................................................286 Chapter 20. Subshells.....................................................................................................................................288 Chapter 21. Restricted Shells.........................................................................................................................291 Chapter 22. Process Substitution...................................................................................................................293 Chapter 23. Functions....................................................................................................................................295 23.1. Complex Functions and Function Complexities.........................................................................297 23.2. Local Variables...........................................................................................................................306 23.2.1. Local variables help make recursion possible...................................................................308 ii
Advanced Bash−Scripting Guide
Table of Contents Chapter 23. Functions 23.3. Recursion Without Local Variables............................................................................................309 Chapter 24. Aliases.........................................................................................................................................311 Chapter 25. List Constructs...........................................................................................................................314 Chapter 26. Arrays.........................................................................................................................................317 Chapter 27. Files.............................................................................................................................................343 Chapter 28. /dev and /proc.............................................................................................................................344 28.1. /dev..............................................................................................................................................344 28.2. /proc............................................................................................................................................345 Chapter 29. Of Zeros and Nulls.....................................................................................................................350 Chapter 30. Debugging...................................................................................................................................354 Chapter 31. Options........................................................................................................................................362 Chapter 32. Gotchas.......................................................................................................................................364 Chapter 33. Scripting With Style..................................................................................................................371 33.1. Unofficial Shell Scripting Stylesheet..........................................................................................371 Chapter 34. Miscellany...................................................................................................................................374 34.1. Interactive and non−interactive shells and scripts......................................................................374 34.2. Shell Wrappers............................................................................................................................375 34.3. Tests and Comparisons: Alternatives..........................................................................................378 34.4. Recursion....................................................................................................................................379 34.5. "Colorizing" Scripts....................................................................................................................381 34.6. Optimizations..............................................................................................................................385 34.7. Assorted Tips..............................................................................................................................386 34.8. Security Issues............................................................................................................................395 34.9. Portability Issues.........................................................................................................................395 34.10. Shell Scripting Under Windows...............................................................................................396 Chapter 35. Bash, version 2...........................................................................................................................397 Chapter 36. Endnotes.....................................................................................................................................402 36.1. Author's Note..............................................................................................................................402 36.2. About the Author........................................................................................................................402 36.3. Where to Go For Help.................................................................................................................402 36.4. Tools Used to Produce This Book..............................................................................................402 36.4.1. Hardware...........................................................................................................................402 36.4.2. Software and Printware.....................................................................................................403 36.5. Credits.........................................................................................................................................403 iii
Advanced Bash−Scripting Guide
Table of Contents Bibliography....................................................................................................................................................405 Appendix A. Contributed Scripts..................................................................................................................411 Appendix B. Reference Cards........................................................................................................................474 Appendix C. A Sed and Awk Micro−Primer................................................................................................479 C.1. Sed................................................................................................................................................479 C.2. Awk..............................................................................................................................................482 Appendix D. Exit Codes With Special Meanings.........................................................................................485 Appendix E. A Detailed Introduction to I/O and I/O Redirection.............................................................486 Appendix F. Standard Command−Line Options.........................................................................................488 Appendix G. Important System Directories.................................................................................................490 Appendix H. Localization...............................................................................................................................491 Appendix I. History Commands....................................................................................................................494 Appendix J. A Sample .bashrc File...............................................................................................................495 Appendix K. Converting DOS Batch Files to Shell Scripts........................................................................506 Appendix L. Exercises....................................................................................................................................510 L.1. Analyzing Scripts.........................................................................................................................510 L.2. Writing Scripts.............................................................................................................................511 Appendix M. Revision History.......................................................................................................................518 Appendix N. To Do List..................................................................................................................................519 Appendix O. Copyright..................................................................................................................................520
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Chapter 1. Why Shell Programming? A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it. Writing shell scripts is not hard to learn, since the scripts can be built in bite−sized sections and there is only a fairly small set of shell−specific operators and options [1] to learn. The syntax is simple and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward. A shell script is a "quick and dirty" method of prototyping a complex application. Getting even a limited subset of the functionality to work in a shell script is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, or Perl. Shell scripting hearkens back to the classic Unix philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high powered all−in−one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool. When not to use shell scripts • Resource−intensive tasks, especially where speed is a factor (sorting, hashing, etc.) • Procedures involving heavy−duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead) • Cross−platform portability required (use C instead) • Complex applications, where structured programming is a necessity (need type−checking of variables, function prototypes, etc.) • Mission−critical applications upon which you are betting the ranch, or the future of the company • Situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism • Project consists of subcomponents with interlocking dependencies • Extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line−by−line fashion) • Need multi−dimensional arrays • Need data structures, such as linked lists or trees • Need to generate or manipulate graphics or GUIs • Need direct access to system hardware • Need port or socket I/O • Need to use libraries or interface with legacy code • Proprietary, closed−source applications (shell scripts put the source code right out in the open for all the world to see) If any of the above applies, consider a more powerful scripting language −− perhaps Perl, Tcl, Python, Ruby −− or possibly a high−level compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step. Chapter 1. Why Shell Programming?
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Advanced Bash−Scripting Guide We will be using Bash, an acronym for "Bourne−Again shell" and a pun on Stephen Bourne's now classic Bourne shell. Bash has become a de facto standard for shell scripting on all flavors of Unix. Most of the principles dealt with in this book apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, [2] and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.) What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work −− they've been tested −− and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (scriptname.sh or scriptname.bash), [3] give them execute permission (chmod u+rx scriptname), then run them to see what happens. Should the source archive not be available, then cut−and−paste from the HTML, pdf, or text rendered versions. Be aware that some of the scripts below introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment. Unless otherwise noted, the author of this book wrote the example scripts that follow.
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Chapter 2. Starting Off With a Sha−Bang In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked.
Example 2−1. cleanup: A script to clean up the log files in /var/log # Cleanup # Run as root, of course. cd /var/log cat /dev/null > messages cat /dev/null > wtmp echo "Logs cleaned up."
There is nothing unusual here, just a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script can easily be modified, customized, or generalized for a particular application.
Example 2−2. cleanup: An improved clean−up script #!/bin/bash # Proper header for a Bash script. # Cleanup, version 2 # Run as root, of course. # Insert code here to print error message and exit if not root. LOG_DIR=var/log # Variables are better than hard−coded values. cd $LOG_DIR cat /dev/null > messages cat /dev/null > wtmp
echo "Logs cleaned up." exit # The right and proper method of "exiting" from a script.
Now that's beginning to look like a real script. But we can go even farther . . .
Example 2−3. cleanup: An enhanced and generalized version of above scripts. #!/bin/bash # Cleanup, version 3 # # #
Warning: −−−−−−− This script uses quite a number of features that will be explained
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Advanced Bash−Scripting Guide #+ later on. # By the time you've finished the first half of the book, #+ there should be nothing mysterious about it.
LOG_DIR=/var/log ROOT_UID=0 # LINES=50 # E_XCD=66 # E_NOTROOT=67 #
Only users with $UID 0 have root privileges. Default number of lines saved. Can't change directory? Non−root exit error.
# Run as root, of course. if [ "$UID" −ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi if [ −n "$1" ] # Test if command line argument present (non−empty). then lines=$1 else lines=$LINES # Default, if not specified on command line. fi
# #+ #+ # # # # # # # # # #*
Stephane Chazelas suggests the following, as a better way of checking command line arguments, but this is still a bit advanced for this stage of the tutorial. E_WRONGARGS=65 case "$1" "" ) *[!0−9]*) * ) esac
# Non−numerical argument (bad arg format)
in lines=50;; echo "Usage: `basename $0` file−to−cleanup"; exit $E_WRONGARGS;; lines=$1;;
Skip ahead to "Loops" chapter to decipher all this.
cd $LOG_DIR if [ `pwd` != "$LOG_DIR" ]
# or if [ "$PWD" != "$LOG_DIR" ] # Not in /var/log?
then echo "Can't change to $LOG_DIR." exit $E_XCD fi # Doublecheck if in right directory, before messing with log file. # far more efficient is: # # cd /var/log || { # echo "Cannot change to necessary directory." >&2 # exit $E_XCD; # }
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Advanced Bash−Scripting Guide tail −$lines messages > mesg.temp # Saves last section of message log file. mv mesg.temp messages # Becomes new log directory.
# cat /dev/null > messages #* No longer needed, as the above method is safer. cat /dev/null > wtmp # echo "Logs cleaned up."
': > wtmp' and '> wtmp'
have the same effect.
exit 0 # A zero return value from the script upon exit #+ indicates success to the shell.
Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of the message log intact. You will constantly discover ways of refining previously written scripts for increased effectiveness. The sha−bang ( #!) at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two−byte [4] "magic number", a special marker that designates a file type, or in this case an executable shell script (see man magic for more details on this fascinating topic). Immediately following the sha−bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (line 1 of the script), ignoring comments. [5] #!/bin/sh #!/bin/bash #!/usr/bin/perl #!/usr/bin/tcl #!/bin/sed −f #!/usr/awk −f
Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. [6] Using #!/bin/sh, the default Bourne shell in most commercial variants of Unix, makes the script portable to non−Linux machines, though you may have to sacrifice a few Bash−specific features. The script will, however, conform to the POSIX [7] sh standard. Note that the path given at the "sha−bang" must be correct, otherwise an error message −− usually "Command not found" −− will be the only result of running the script. #! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell−specific construct. Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine. This tutorial encourages a modular approach to constructing a script. Make note of and collect "boilerplate" code snippets that might be useful in future scripts. Eventually you can build a quite extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters. if [ $# −ne $Number_of_expected args ] then
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Advanced Bash−Scripting Guide echo "Usage: `basename $0` script_parameters" exit $E_WRONG_ARGS fi
2.1. Invoking the script Having written the script, you can invoke it by sh scriptname, [8] or alternatively bash scriptname. (Not recommended is using sh <scriptname, since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod. Either: chmod 555 scriptname (gives everyone read/execute permission) [9] or chmod +rx scriptname (gives everyone read/execute permission) chmod u+rx scriptname (gives only the script owner read/execute permission) Having made the script executable, you may now test it by ./scriptname. [10] If it begins with a "sha−bang" line, invoking the script calls the correct command interpreter to run it. As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a system−wide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command line.
2.2. Preliminary Exercises 1. System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful. 2. Write a script that upon invocation shows the time and date, lists all logged−in users, and gives the system uptime. The script then saves this information to a logfile.
Chapter 2. Starting Off With a Sha−Bang
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Part 2. Basics Table of Contents 3. Special Characters 4. Introduction to Variables and Parameters 4.1. Variable Substitution 4.2. Variable Assignment 4.3. Bash Variables Are Untyped 4.4. Special Variable Types 5. Quoting 6. Exit and Exit Status 7. Tests 7.1. Test Constructs 7.2. File test operators 7.3. Other Comparison Operators 7.4. Nested if/then Condition Tests 7.5. Testing Your Knowledge of Tests 8. Operations and Related Topics 8.1. Operators 8.2. Numerical Constants
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Chapter 3. Special Characters Special Characters Found In Scripts and Elsewhere # Comments. Lines beginning with a # (with the exception of #!) are comments. # This line is a comment.
Comments may also occur at the end of a command. echo "A comment will follow." # Comment here.
Comments may also follow whitespace at the beginning of a line. # A tab precedes this comment.
A command may not follow a comment on the same line. There is no method of terminating the comment, in order for "live code" to begin on the same line. Use a new line for the next command. Of course, an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter substitution constructs and in numerical constant expressions. echo echo echo echo
"The # here does not begin a comment." 'The # here does not begin a comment.' The \# here does not begin a comment. The # here begins a comment.
echo ${PATH#*:} echo $(( 2#101011 ))
# Parameter substitution, not a comment. # Base conversion, not a comment.
# Thanks, S.C.
The standard quoting and escape characters (" ' \) escape the #. Certain pattern matching operations also use the #. ; Command separator [semicolon]. Permits putting two or more commands on the same line. echo hello; echo there
if [ −x "$filename" ]; then
# Note that "if" and "then" need separation. # Why? echo "File $filename exists."; cp $filename $filename.bak else echo "File $filename not found."; touch $filename fi; echo "File test complete."
Note that the ";" sometimes needs to be escaped. Chapter 3. Special Characters
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Advanced Bash−Scripting Guide ;; Terminator in a case option [double semicolon]. case "$variable" in abc) echo "\$variable = abc" ;; xyz) echo "\$variable = xyz" ;; esac
. "dot" command [period]. Equivalent to source (see Example 11−19). This is a bash builtin. . "dot", as a component of a filename. When working with filenames, a dot is the prefix of a "hidden" file, a file that an ls will not normally show. bash$ touch .hidden−file bash$ ls −l total 10 −rw−r−−r−− 1 bozo −rw−r−−r−− 1 bozo −rw−r−−r−− 1 bozo
bash$ ls −al total 14 drwxrwxr−x drwx−−−−−− −rw−r−−r−− −rw−r−−r−− −rw−r−−r−− −rw−rw−r−−
2 52 1 1 1 1
bozo bozo bozo bozo bozo bozo
bozo bozo bozo bozo bozo bozo
4034 Jul 18 22:04 data1.addressbook 4602 May 25 13:58 data1.addressbook.bak 877 Dec 17 2000 employment.addressbook
1024 3072 4034 4602 877 0
Aug Aug Jul May Dec Aug
29 29 18 25 17 29
20:54 20:51 22:04 13:58 2000 20:54
./ ../ data1.addressbook data1.addressbook.bak employment.addressbook .hidden−file
When considering directory names, a single dot represents the current working directory, and two dots denote the parent directory. bash$ pwd /home/bozo/projects bash$ cd . bash$ pwd /home/bozo/projects bash$ cd .. bash$ pwd /home/bozo/
The dot often appears as the destination (directory) of a file movement command. bash$ cp /home/bozo/current_work/junk/* .
. "dot" character match. When matching characters, as part of a regular expression, a "dot" matches a single character. "
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Advanced Bash−Scripting Guide partial quoting [double quote]. "STRING" preserves (from interpretation) most of the special characters within STRING. See also Chapter 5. ' full quoting [single quote]. 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than using ". See also Chapter 5. , comma operator. The comma operator links together a series of arithmetic operations. All are evaluated, but only the last one is returned. let "t2 = ((a = 9, 15 / 3))"
# Set "a" and calculate "t2".
\ escape [backslash]. A quoting mechanism for single characters. \X "escapes" the character X. This has the effect of "quoting" X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally. See Chapter 5 for an in−depth explanation of escaped characters. / Filename path separator [forward slash]. Separates the components of a filename (as in /home/bozo/projects/Makefile). This is also the division arithmetic operator. ` command substitution. The `command` construct makes available the output of command for setting a variable. This is also known as backticks or backquotes. : null command [colon]. This is the shell equivalent of a "NOP" (no op, a do−nothing operation). It may be considered a synonym for the shell builtin true. The ":" command is a itself a Bash builtin, and its exit status is "true" (0). : echo $?
# 0
Endless loop: while : do operation−1 operation−2 ... operation−n done # Same as: # while true # do # ... # done
Placeholder in if/then test: if condition
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Advanced Bash−Scripting Guide then : # Do nothing and branch ahead else take−some−action fi
Provide a placeholder where a binary operation is expected, see Example 8−2 and default parameters. : ${username=`whoami`} # ${username=`whoami`} #
without the leading : gives an error unless "username" is a command or builtin...
Provide a placeholder where a command is expected in a here document. See Example 17−10. Evaluate string of variables using parameter substitution (as in Example 9−13). : ${HOSTNAME?} ${USER?} ${MAIL?} #Prints error message if one or more of essential environmental variables not set.
Variable expansion / substring replacement. In combination with the > redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it. : > data.xxx
# File "data.xxx" now empty.
# Same effect as cat /dev/null >data.xxx # However, this does not fork a new process, since ":" is a builtin.
See also Example 12−14. In combination with the >> redirection operator, has no effect on a pre−existing target file (: >> target_file). If the file did not previously exist, creates it. This applies to regular files, not pipes, symlinks, and certain special files. May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may be appear in a comment. However, this is not the case with :. : This is a comment that generates an error, ( if [ $x −eq 3] ).
The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable. bash$ echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games
! reverse (or negate) the sense of a test or exit status. The ! operator inverts the exit status of the command to which it is applied (see Example 6−2). It also inverts the meaning of a test operator. This can, for example, change the sense of "equal" ( = ) to "not−equal" ( != ). The ! operator is a Bash keyword.
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Advanced Bash−Scripting Guide In a different context, the ! also appears in indirect variable references. In yet another context, from the command line, the ! invokes the Bash history mechanism (see Appendix I). Note that within a script, the history mechanism is disabled. * wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By itself, it matches every filename in a given directory. bash$ echo * abs−book.sgml add−drive.sh agram.sh alias.sh
The * also represents any number (or zero) characters in a regular expression. * arithmetic operator. In the context of arithmetic operations, the * denotes multiplication. A double asterisk, **, is the exponentiation operator. ? test operator. Within certain expressions, the ? indicates a test for a condition. In a double parentheses construct, the ? serves as a C−style trinary operator. See Example 9−29. In a parameter substitution expression, the ? tests whether a variable has been set. ? wild card. The ? character serves as a single−character "wild card" for filename expansion in globbing, as well as representing one character in an extended regular expression. $ Variable substitution. var1=5 var2=23skidoo echo $var1 echo $var2
# 5 # 23skidoo
A $ prefixing a variable name indicates the value the variable holds. $ end−of−line. In a regular expression, a "$" addresses the end of a line of text. ${} Parameter substitution. $*, $@ positional parameters. $? exit status variable. The $? variable holds the exit status of a command, a function, or of the script itself. $$ process ID variable. The $$ variable holds the process ID of the script in which it appears. () command group. (a=hello; echo $a)
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Advanced Bash−Scripting Guide A listing of commands within parentheses starts a subshell. Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell. a=123 ( a=321; ) echo "a = $a" # a = 123 # "a" within parentheses acts like a local variable.
array initialization. Array=(element1 element2 element3)
{xxx,yyy,zzz,...} Brace expansion. grep Linux file*.{txt,htm*} # Finds all instances of the word "Linux" # in the files "fileA.txt", "file2.txt", "fileR.html", "file−87.htm", etc.
A command may act upon a comma−separated list of file specs within braces. [11] Filename expansion (globbing) applies to the file specs between the braces. No spaces allowed within the braces unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C {} Block of code [curly brackets]. Also referred to as an "inline group", this construct, in effect, creates an anonymous function. However, unlike a function, the variables in a code block remain visible to the remainder of the script. bash$ { local a; a=123; } bash: local: can only be used in a function
a=123 { a=321; } echo "a = $a"
# a = 321
(value inside code block)
# Thanks, S.C.
The code block enclosed in braces may have I/O redirected to and from it.
Example 3−1. Code blocks and I/O redirection Chapter 3. Special Characters
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Advanced Bash−Scripting Guide #!/bin/bash # Reading lines in /etc/fstab. File=/etc/fstab { read line1 read line2 } < $File echo echo echo echo echo
"First line in $File is:" "$line1" "Second line in $File is:" "$line2"
exit 0
Example 3−2. Saving the results of a code block to a file #!/bin/bash # rpm−check.sh # Queries an rpm file for description, listing, and whether it can be installed. # Saves output to a file. # # This script illustrates using a code block. SUCCESS=0 E_NOARGS=65 if [ −z "$1" ] then echo "Usage: `basename $0` rpm−file" exit $E_NOARGS fi { echo echo "Archive Description:" rpm −qpi $1 # Query description. echo echo "Archive Listing:" rpm −qpl $1 # Query listing. echo rpm −i −−test $1 # Query whether rpm file can be installed. if [ "$?" −eq $SUCCESS ] then echo "$1 can be installed." else echo "$1 cannot be installed." fi echo } > "$1.test" # Redirects output of everything in block to file. echo "Results of rpm test in file $1.test" # See rpm man page for explanation of options. exit 0
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Advanced Bash−Scripting Guide Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell. [12] {} \; pathname. Mostly used in find constructs. This is not a shell builtin. The ";" ends the −exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell. [] test. Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a link to the external command /usr/bin/test. [[ ]] test. Test expression between [[ ]] (shell keyword). See the discussion on the [[ ... ]] construct. [] array element. In the context of an array, brackets set off the numbering of each element of that array. Array[1]=slot_1 echo ${Array[1]}
[] range of characters. As part of a regular expression, brackets delineate a range of characters to match. (( )) integer expansion. Expand and evaluate integer expression between (( )). See the discussion on the (( ... )) construct. > &> >& >> < redirection. scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists. command &>filename redirects both the stdout and the stderr of command to filename. command >&2 redirects stdout of command to stderr. scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it will be created. process substitution.
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Advanced Bash−Scripting Guide (command)> | force redirection (even if the noclobber option is set). This will forcibly overwrite an existing file. || OR logical operator. In a test construct, the || operator causes a return of 0 (success) if either of the linked test conditions is true. & Run job in background. A command followed by an & will run in the background. bash$ sleep 10 & [1] 850 [1]+ Done
sleep 10
Within a script, commands and even loops may run in the background.
Example 3−3. Running a loop in the background #!/bin/bash # background−loop.sh for i in 1 2 3 4 5 6 7 8 9 10 # First loop. do echo −n "$i " done & # Run this loop in background. # Will sometimes execute after second loop. echo
# This 'echo' sometimes will not display.
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Advanced Bash−Scripting Guide for i in 11 12 13 14 15 16 17 18 19 20 do echo −n "$i " done echo
# Second loop.
# This 'echo' sometimes will not display.
# ====================================================== # The expected output from the script: # 1 2 3 4 5 6 7 8 9 10 # 11 12 13 14 15 16 17 18 19 20 # # # #
Sometimes, though, you get: 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 bozo $ (The second 'echo' doesn't execute. Why?)
# Occasionally also: # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # (The first 'echo' doesn't execute. Why?) # Very rarely something like: # 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20 # The foreground loop preempts the background one. exit 0
A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this. && AND logical operator. In a test construct, the && operator causes a return of 0 (success) only if both the linked test conditions are true. − option, prefix. Option flag for a command or filter. Prefix for an operator. COMMAND −[Option1][Option2][...] ls −al sort −dfu $filename set −− $variable if [ $file1 −ot $file2 ] then echo "File $file1 is older than $file2." fi if [ "$a" −eq "$b" ] then echo "$a is equal to $b." fi if [ "$c" −eq 24 −a "$d" −eq 47 ] then echo "$c equals 24 and $d equals 47."
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Advanced Bash−Scripting Guide fi
− redirection from/to stdin or stdout [dash]. (cd /source/directory && tar cf − . ) | (cd /dest/directory && tar xpvf −) # Move entire file tree from one directory to another # [courtesy Alan Cox , with a minor change] # # # # # # # # # # # # # # #
1) cd /source/directory 2) && 3) tar cf − .
4) 5) 6) 7) 8)
| ( ... ) cd /dest/directory && tar xpvf −
Source directory, where the files to be moved are. "And−list": if the 'cd' operation successful, then execute the The 'c' option 'tar' archiving command creates a new archive, the 'f' (file) option, followed by '−' designates the target and do it in current directory tree ('.'). Piped to... a subshell Change to the destination directory. "And−list", as above Unarchive ('x'), preserve ownership and file permissions ('p' and send verbose messages to stdout ('v'), reading data from stdin ('f' followed by '−'). Note that 'x' is a command, and 'p', 'v', 'f' are options.
Whew!
# More elegant than, but equivalent to: # cd source−directory # tar cf − . | (cd ../target−directory; tar xzf −) # # cp −a /source/directory /dest also has same effect. bunzip2 linux−2.4.3.tar.bz2 | tar xvf − # −−uncompress tar file−− | −−then pass it to "tar"−− # If "tar" has not been patched to handle "bunzip2", # this needs to be done in two discrete steps, using a pipe. # The purpose of the exercise is to unarchive "bzipped" kernel source.
Note that in this context the "−" is not itself a Bash operator, but rather an option recognized by certain Unix utilities that write to stdout, such as tar, cat, etc. bash$ echo "whatever" | cat − whatever
Where a filename is expected, − redirects output to stdout (sometimes seen with tar cf), or accepts input from stdin, rather than from a file. This is a method of using a file−oriented utility as a filter in a pipe. bash$ file Usage: file [−bciknvzL] [−f namefile] [−m magicfiles] file...
By itself on the command line, file fails with an error message. Add a "−" for a more useful result. This causes the shell to await user input.
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Advanced Bash−Scripting Guide bash$ file − abc standard input:
ASCII text
bash$ file − #!/bin/bash standard input:
Bourne−Again shell script text executable
Now the command accepts input from stdin and analyzes it. The "−" can be used to pipe stdout to other commands. This permits such stunts as prepending lines to a file. Using diff to compare a file with a section of another: grep Linux file1 | diff file2 − Finally, a real−world example using − with tar.
Example 3−4. Backup of all files changed in last day #!/bin/bash # Backs up all files in current directory modified within last 24 hours #+ in a "tarball" (tarred and gzipped file). BACKUPFILE=backup−$(date +%m−%d−%Y) # Embeds date in backup filename. # Thanks, Joshua Tschida, for the idea. archive=${1:−$BACKUPFILE} # If no backup−archive filename specified on command line, #+ it will default to "backup−MM−DD−YYYY.tar.gz." tar cvf − `find . −mtime −1 −type f −print` > $archive.tar gzip $archive.tar echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."
# Stephane Chazelas points out that the above code will fail #+ if there are too many files found #+ or if any filenames contain blank characters. # He suggests the following alternatives: # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # find . −mtime −1 −type f −print0 | xargs −0 tar rvf "$archive.tar" # using the GNU version of "find".
# find . −mtime −1 −type f −exec tar rvf "$archive.tar" '{}' \; # portable to other UNIX flavors, but much slower. # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
exit 0
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Advanced Bash−Scripting Guide Filenames beginning with "−" may cause problems when coupled with the "−" redirection operator. A script should check for this and add an appropriate prefix to such filenames, for example ./−FILENAME, $PWD/−FILENAME, or $PATHNAME/−FILENAME. If the value of a variable begins with a −, this may likewise create problems. var="−n" echo $var # Has the effect of "echo −n", and outputs nothing.
− previous working directory. A cd − command changes to the previous working directory. This uses the $OLDPWD environmental variable. Do not confuse the "−" used in this sense with the "−" redirection operator just discussed. The interpretation of the "−" depends on the context in which it appears. − Minus. Minus sign in an arithmetic operation. = Equals. Assignment operator a=28 echo $a
# 28
In a different context, the "=" is a string comparison operator. + Plus. Addition arithmetic operator. In a different context, the + is a Regular Expression operator. + Option. Option flag for a command or filter. Certain commands and builtins use the + to enable certain options and the − to disable them. % modulo. Modulo (remainder of a division) arithmetic operation. In a different context, the % is a pattern matching operator. ~ home directory [tilde]. This corresponds to the $HOME internal variable. ~bozo is bozo's home directory, and ls ~bozo lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the contents of it. bash$ echo ~bozo /home/bozo bash$ echo ~ /home/bozo bash$ echo ~/ /home/bozo/ bash$ echo ~: /home/bozo:
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Advanced Bash−Scripting Guide bash$ echo ~nonexistent−user ~nonexistent−user
~+ current working directory. This corresponds to the $PWD internal variable. ~− previous working directory. This corresponds to the $OLDPWD internal variable. ^ beginning−of−line. In a regular expression, a "^" addresses the beginning of a line of text. Control Characters change the behavior of the terminal or text display. A control character is a CONTROL + key combination. ◊ Ctl−B Backspace (nondestructive). ◊ Ctl−C ◊
Break. Terminate a foreground job. Ctl−D Log out from a shell (similar to exit).
"EOF" (end of file). This also terminates input from stdin. ◊ Ctl−G "BEL" (beep). ◊ Ctl−H "Rubout" (destructive backspace). #!/bin/bash # Embedding Ctl−H in a string. a="^H^H" echo "abcdef" echo −n "abcdef$a " # Space at end ^ echo −n "abcdef$a" # No space at end
# Two Ctl−H's (backspaces). # abcdef # abcd f ^ Backspaces twice. # abcdef Doesn't backspace (why?). # Results may not be quite as expected.
echo; echo
◊ Ctl−I Horizontal tab. ◊ Ctl−J Newline (line feed). ◊ Ctl−K
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Advanced Bash−Scripting Guide Vertical tab. ◊ Ctl−L Formfeed (clear the terminal screen). This has the same effect as the clear command. ◊ Ctl−M Carriage return. #!/bin/bash # Thank you, Lee Maschmeyer, for this example.
read −n 1 −s −p $'Control−M leaves cursor at beginning of this line. Press Enter. \x # Of course, '0d' is the hex equivalent of Control echo >&2 # The '−s' makes anything typed silent, #+ so it is necessary to go to new line explicitly. read −n 1 −s −p $'Control−J leaves cursor on next line. \x0a' echo >&2 # Control−J is linefeed. ### read −n 1 −s −p $'And Control−K\x0bgoes straight down.' echo >&2 # Control−K is vertical tab. # A better example of the effect of a vertical tab is: var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a' echo "$var" # This works the same way as the above example. However: echo "$var" | col # This causes the right end of the line to be higher than the left end. # It also explains why we started and ended with a line feed −− #+ to avoid a garbled screen. # As Lee Maschmeyer explains: # −−−−−−−−−−−−−−−−−−−−−−−−−− # In the [first vertical tab example] . . . the vertical tab #+ makes the printing go straight down without a carriage return. # This is true only on devices, such as the Linux console, #+ that can't go "backward." # The real purpose of VT is to go straight UP, not down. # It can be used to print superscripts on a printer. # The col utility can be used to emulate the proper behavior of VT. exit 0
◊ Ctl−Q Resume (XON). This resumes stdin in a terminal. ◊ Ctl−S Suspend (XOFF). This freezes stdin in a terminal. (Use Ctl−Q to restore input.) ◊ Ctl−U
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Advanced Bash−Scripting Guide Erase a line of input. ◊ Ctl−Z Pause a foreground job. Whitespace functions as a separator, separating commands or variables. Whitespace consists of either spaces, tabs, blank lines, or any combination thereof. In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error. Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections. $IFS, the special variable separating fields of input to certain commands, defaults to whitespace.
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24
Chapter 4. Introduction to Variables and Parameters Variables are at the heart of every programming and scripting language. They appear in arithmetic operations and manipulation of quantities, string parsing, and are indispensable for working in the abstract with symbols − tokens that represent something else. A variable is nothing more than a location or set of locations in computer memory holding an item of data.
4.1. Variable Substitution The name of a variable is a placeholder for its value, the data it holds. Referencing its value is called variable substitution. $ Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. The only time a variable appears "naked", without the $ prefix, is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see Example 30−5). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3). Enclosing a referenced value in double quotes (" ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as "weak quoting". Using single quotes (' ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as "strong quoting". See Chapter 5 for a detailed discussion. Note that $variable is actually a simplified alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see Section 9.3, below).
Example 4−1. Variable assignment and substitution #!/bin/bash # Variables: assignment and substitution a=375 hello=$a #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # No space permitted on either side of = sign when initializing variables. # If "VARIABLE =value", #+ script tries to run "VARIABLE" command with one argument, "=value". # If "VARIABLE= value", #+ script tries to run "value" command with #+ the environmental variable "VARIABLE" set to "". #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
echo hello
# Not a variable reference, just the string "hello".
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Advanced Bash−Scripting Guide echo $hello echo ${hello} # Identical to above. echo "$hello" echo "${hello}" echo hello="A B C D" echo $hello # A B C D echo "$hello" # A B C D # As you see, echo $hello and echo "$hello" # Quoting a variable preserves whitespace.
give different results.
echo echo '$hello' # $hello # Variable referencing disabled by single quotes, #+ which causes the "$" to be interpreted literally. # Notice the effect of different types of quoting.
hello= # Setting it to a null value. echo "\$hello (null value) = $hello" # Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below). # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # It is permissible to set multiple variables on the same line, #+ if separated by white space. # Caution, this may reduce legibility, and may not be portable. var1=variable1 var2=variable2 var3=variable3 echo echo "var1=$var1 var2=$var2 var3=$var3" # May cause problems with older versions of "sh". # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− echo; echo numbers="one two three" other_numbers="1 2 3" # If whitespace within a variable, then quotes necessary. echo "numbers = $numbers" echo "other_numbers = $other_numbers" # other_numbers = 1 2 3 echo echo "uninitialized_variable = $uninitialized_variable" # Uninitialized variable has null value (no value at all). uninitialized_variable= # Declaring, but not initializing it #+ (same as setting it to a null value, as above). echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. uninitialized_variable=23 # Set it. unset uninitialized_variable # Unset it. echo "uninitialized_variable = $uninitialized_variable" # It still has a null value.
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Advanced Bash−Scripting Guide echo exit 0
An uninitialized variable has a "null" value − no assigned value at all (not zero!). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable. echo "$uninitialized" let "uninitialized += 5" echo "$uninitialized" # # #+ #
# (blank line) # Add 5 to it. # 5
Conclusion: An uninitialized variable has no value, however it acts as if it were 0 in an arithmetic operation. This is undocumented (and probably non−portable) behavior.
See also Example 11−20.
4.2. Variable Assignment = the assignment operator (no space before & after) Do not confuse this with = and −eq, which test, rather than assign! Note that = can be either an assignment or a test operator, depending on context.
Example 4−2. Plain Variable Assignment #!/bin/bash # Naked variables echo # When is a variable "naked", i.e., lacking the '$' in front? # When it is being assigned, rather than referenced. # Assignment a=879 echo "The value of \"a\" is $a." # Assignment using 'let' let a=16+5 echo "The value of \"a\" is now $a." echo # In a 'for' loop (really, a type of disguised assignment) echo −n "Values of \"a\" in the loop are: " for a in 7 8 9 11
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Advanced Bash−Scripting Guide do echo −n "$a " done echo echo # In echo read echo
a 'read' statement (also a type of assignment) −n "Enter \"a\" " a "The value of \"a\" is now $a."
echo exit 0
Example 4−3. Variable Assignment, plain and fancy #!/bin/bash a=23 echo $a b=$a echo $b
# Simple case
# Now, getting a little bit fancier (command substitution). a=`echo Hello!` # Assigns result of 'echo' command to 'a' echo $a # Note that using an exclamation mark (!) in command substitution #+ will not work from the command line, #+ since this triggers the Bash "history mechanism." # Within a script, however, the history functions are disabled. a=`ls −l` echo $a echo echo "$a"
# Assigns result of 'ls −l' command to 'a' # Unquoted, however, removes tabs and newlines. # The quoted variable preserves whitespace. # (See the chapter on "Quoting.")
exit 0
Variable assignment using the $(...) mechanism (a newer method than backquotes) # From /etc/rc.d/rc.local R=$(cat /etc/redhat−release) arch=$(uname −m)
4.3. Bash Variables Are Untyped Unlike many other programming languages, Bash does not segregate its variables by "type". Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.
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Advanced Bash−Scripting Guide Example 4−4. Integer or string? #!/bin/bash # int−or−string.sh: Integer or string? a=2334 let "a += 1" echo "a = $a " echo
# Integer.
b=${a/23/BB}
# # # # #
echo "b = $b" declare −i b echo "b = $b" let "b += 1" echo "b = $b" echo c=BB34 echo "c = $c" d=${c/BB/23} echo "d = $d" let "d += 1" echo "d = $d" echo
# a = 2335 # Integer, still.
Substitute "BB" for "23". This transforms $b into a string. b = BB35 Declaring it an integer doesn't help. b = BB35
# BB35 + 1 = # b = 1
# # # # # #
c = BB34 Substitute "23" for "BB". This makes $d an integer. d = 2334 2334 + 1 = d = 2335
# What about null variables? e="" echo "e = $e" # e = let "e += 1" # Arithmetic operations allowed on a null variable? echo "e = $e" # e = 1 echo # Null variable transformed into an integer. # What about undeclared variables? echo "f = $f" # f = let "f += 1" # Arithmetic operations allowed? echo "f = $f" # f = 1 echo # Undeclared variable transformed into an integer.
# Variables in Bash are essentially untyped. exit 0
Untyped variables are both a blessing and a curse. They permit more flexibility in scripting (enough rope to hang yourself!) and make it easier to grind out lines of code. However, they permit errors to creep in and encourage sloppy programming habits. The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for you.
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Advanced Bash−Scripting Guide
4.4. Special Variable Types local variables variables visible only within a code block or function (see also local variables in functions) environmental variables variables that affect the behavior of the shell and user interface In a more general context, each process has an "environment", that is, a group of variables that hold information that the process may reference. In this sense, the shell behaves like any other process. Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment. The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems. bash$ eval "`seq 10000 | sed −e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`" bash$ du bash: /usr/bin/du: Argument list too long
(Thank you, S. C. for the clarification, and for providing the above example.) If a script sets environmental variables, they need to be "exported", that is, reported to the environment local to the script. This is the function of the export command.
A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command line cannot export variables back to the command line environment. Child processes cannot export variables back to the parent processes that spawned them. −−− positional parameters arguments passed to the script from the command line − $0, $1, $2, $3... $0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. [13] After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}. The special variables $* and $@ denote all the positional parameters.
Example 4−5. Positional Parameters #!/bin/bash # Call this script with at least 10 parameters, for example # ./scriptname 1 2 3 4 5 6 7 8 9 10
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Advanced Bash−Scripting Guide MINPARAMS=10 echo echo "The name of this script is \"$0\"." # Adds ./ for current directory echo "The name of this script is \"`basename $0`\"." # Strips out path name info (see 'basename') echo if [ −n "$1" ] then echo "Parameter #1 is $1" fi
# Tested variable is quoted. # Need quotes to escape #
if [ −n "$2" ] then echo "Parameter #2 is $2" fi if [ −n "$3" ] then echo "Parameter #3 is $3" fi # ...
if [ −n "${10}" ] # Parameters > $9 must be enclosed in {brackets}. then echo "Parameter #10 is ${10}" fi echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" echo "All the command−line parameters are: "$*"" if [ $# −lt "$MINPARAMS" ] then echo echo "This script needs at least $MINPARAMS command−line arguments!" fi echo exit 0
The bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command line. This also requires indirect referencing. args=$# lastarg=${!args}
# Number of args passed. # Note that lastarg=${!$#} doesn't work.
Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There must also exist symbolic links to all the alternate names of the script. See Example 12−2. If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent Chapter 4. Introduction to Variables and Parameters
31
Advanced Bash−Scripting Guide this is to append an extra character to both sides of the assignment statement using the expected positional parameter. variable1_=$1_ # This will prevent an error, even if positional parameter is absent. critical_argument01=$variable1_ # The extra character can be stripped off later, if desired, like so. variable1=${variable1_/_/} # Side effects only if $variable1_ begins with "_". # This uses one of the parameter substitution templates discussed in Chapter 9. # Leaving out the replacement pattern results in a deletion. # A more straightforward way of dealing with this is #+ to simply test whether expected positional parameters have been passed. if [ −z $1 ] then exit $POS_PARAMS_MISSING fi
−−−
Example 4−6. wh, whois domain name lookup #!/bin/bash # Does a 'whois domain−name' lookup on any of 3 alternate servers: # ripe.net, cw.net, radb.net # Place this script, named 'wh' in /usr/local/bin # # # #
Requires symbolic links: ln −s /usr/local/bin/wh /usr/local/bin/wh−ripe ln −s /usr/local/bin/wh /usr/local/bin/wh−cw ln −s /usr/local/bin/wh /usr/local/bin/wh−radb
if [ −z "$1" ] then echo "Usage: `basename $0` [domain−name]" exit 65 fi case `basename $0` in # Checks script name and calls proper server "wh" ) whois
[email protected];; "wh−ripe") whois
[email protected];; "wh−radb") whois
[email protected];; "wh−cw" ) whois
[email protected];; * ) echo "Usage: `basename $0` [domain−name]";; esac exit 0
−−−
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Advanced Bash−Scripting Guide The shift command reassigns the positional parameters, in effect shifting them to the left one notch. $1 = is greater than or equal to (within double parentheses) (("$a" >= "$b")) string comparison = is equal to if [ "$a" = "$b" ] == is equal to if [ "$a" == "$b" ] This is a synonym for =. The == comparison operator behaves differently within a double−brackets test than within single brackets. [[ $a == z* ]] [[ $a == "z*" ]]
# True if $a starts with an "z" (pattern matching). # True if $a is equal to z* (literal matching).
[ $a == z* ] [ "$a" == "z*" ]
# File globbing and word splitting take place. # True if $a is equal to z* (literal matching).
# Thanks, Stephane Chazelas
!= is not equal to if [ "$a" != "$b" ] This operator uses pattern matching within a [[ ... ]] construct. < is less than, in ASCII alphabetical order if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ] Note that the "" needs to be escaped within a [ ] construct. Chapter 7. Tests
52
Advanced Bash−Scripting Guide See Example 26−11 for an application of this comparison operator. −z string is "null", that is, has zero length −n string is not "null". The −n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! −z, or even just the unquoted string alone within test brackets (see Example 7−6) normally works, however, this is an unsafe practice. Always quote a tested string. [18]
Example 7−5. Arithmetic and string comparisons #!/bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed. # Bash permits integer operations and comparisons on variables #+ whose value consists of all−integer characters. # Caution advised. echo if [ "$a" −ne "$b" ] then echo "$a is not equal to $b" echo "(arithmetic comparison)" fi echo if [ "$a" != "$b" ] then echo "$a is not equal to $b." echo "(string comparison)" # "4" != "5" # ASCII 52 != ASCII 53 fi # In this particular instance, both "−ne" and "!=" work. echo exit 0
Example 7−6. Testing whether a string is null #!/bin/bash # str−test.sh: Testing null strings and unquoted strings, #+ but not strings and sealing wax, not to mention cabbages and kings . . .
Chapter 7. Tests
53
Advanced Bash−Scripting Guide # Using
if [ ... ]
# If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero). if [ −n $string1 ] # $string1 has not been declared or initialized. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Wrong result. # Shows $string1 as not null, although it was not initialized.
echo
# Lets try it again. if [ −n "$string1" ] # This time, $string1 is quoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Quote strings within test brackets!
echo
if [ $string1 ] # This time, $string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # This works fine. # The [ ] test operator alone detects whether the string is null. # However it is good practice to quote it ("$string1"). # # As Stephane Chazelas points out, # if [ $string1 ] has one argument, "]" # if [ "$string1" ] has two arguments, the empty "$string1" and "]"
echo
string1=initialized if [ $string1 ] # Again, $string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Again, gives correct result.
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Advanced Bash−Scripting Guide # Still, it is better to quote it ("$string1"), because . . .
string1="a = b" if [ $string1 ] # Again, $string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Not quoting "$string1" now gives wrong result! exit 0 # Thank you, also, Florian Wisser, for the "heads−up".
Example 7−7. zmost #!/bin/bash #View gzipped files with 'most' NOARGS=65 NOTFOUND=66 NOTGZIP=67 if [ $# −eq 0 ] # same effect as: if [ −z "$1" ] # $1 can exist, but be empty: zmost "" arg2 arg3 then echo "Usage: `basename $0` filename" >&2 # Error message to stderr. exit $NOARGS # Returns 65 as exit status of script (error code). fi filename=$1 if [ ! −f "$filename" ] # Quoting $filename allows for possible spaces. then echo "File $filename not found!" >&2 # Error message to stderr. exit $NOTFOUND fi if [ ${filename##*.} != "gz" ] # Using bracket in variable substitution. then echo "File $1 is not a gzipped file!" exit $NOTGZIP fi zcat $1 | most # Uses the file viewer 'most' (similar to 'less'). # Later versions of 'most' have file decompression capabilities. # May substitute 'more' or 'less', if desired.
exit $? # Script returns exit status of pipe. # Actually "exit $?" is unnecessary, as the script will, in any case,
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Advanced Bash−Scripting Guide # return the exit status of the last command executed.
compound comparison −a logical and exp1 −a exp2 returns true if both exp1 and exp2 are true. −o logical or exp1 −o exp2 returns true if either exp1 or exp2 are true. These are similar to the Bash comparison operators && and ||, used within double brackets. [[ condition1 && condition2 ]]
The −o and −a operators work with the test command or occur within single test brackets. if [ "$exp1" −a "$exp2" ]
Refer to Example 8−3 and Example 26−16 to see compound comparison operators in action.
7.4. Nested if/then Condition Tests Condition tests using the if/then construct may be nested. The net result is identical to using the && compound comparison operator above. if [ condition1 ] then if [ condition2 ] then do−something # But only if both "condition1" and "condition2" valid. fi fi
See Example 35−4 for an example of nested if/then condition tests.
7.5. Testing Your Knowledge of Tests The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests, as the following excerpt shows. if [ −f $HOME/.Xclients ]; then exec $HOME/.Xclients elif [ −f /etc/X11/xinit/Xclients ]; then exec /etc/X11/xinit/Xclients else # failsafe settings. Although we should never get here # (we provide fallbacks in Xclients as well) it can't hurt. xclock −geometry 100x100−5+5 & xterm −geometry 80x50−50+150 & if [ −f /usr/bin/netscape −a −f /usr/share/doc/HTML/index.html ]; then
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Advanced Bash−Scripting Guide netscape /usr/share/doc/HTML/index.html & fi fi
Explain the "test" constructs in the above excerpt, then examine the entire file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions.
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Chapter 8. Operations and Related Topics 8.1. Operators assignment variable assignment Initializing or changing the value of a variable = All−purpose assignment operator, which works for both arithmetic and string assignments. var=27 category=minerals
# No spaces allowed after the "=".
Do not confuse the "=" assignment operator with the = test operator. #
= as a test operator
if [ "$string1" = "$string2" ] # if [ "X$string1" = "X$string2" ] is safer, # to prevent an error message should one of the variables be empty. # (The prepended "X" characters cancel out.) then command fi
arithmetic operators + plus − minus * multiplication / division ** exponentiation # Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" echo "z = $z"
# z = 125
% modulo, or mod (returns the remainder of an integer division operation) bash$ echo `expr 5 % 3` 2
This operator finds use in, among other things, generating numbers within a specific range (see Example 9−23 and Example 9−26) and formatting program output (see Example 26−15 and Example Chapter 8. Operations and Related Topics
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Advanced Bash−Scripting Guide A−7). It can even be used to generate prime numbers, (see Example A−17). Modulo turns up surprisingly often in various numerical recipes.
Example 8−1. Greatest common divisor #!/bin/bash # gcd.sh: greatest common divisor # Uses Euclid's algorithm # The "greatest common divisor" (gcd) of two integers #+ is the largest integer that will divide both, leaving no remainder. # # #+ #+ #+ #+ # # #
Euclid's algorithm uses successive division. In each pass, dividend /dev/null
# Compare these methods of checking whether a variable has been set #+ with "set −u" . . .
echo "You will not see this message, because script already terminated." HERE=0 exit $HERE
# Will NOT exit here.
Example 9−14. Parameter substitution and "usage" messages #!/bin/bash # usage−message.sh : ${1?"Usage: $0 ARGUMENT"} # Script exits here if command−line parameter absent, #+ with following error message. # usage−message.sh: 1: Usage: usage−message.sh ARGUMENT echo "These two lines echo only if command−line parameter given." echo "command line parameter = \"$1\"" exit 0
# Will exit here only if command−line parameter present.
# Check the exit status, both with and without command−line parameter. # If command−line parameter present, then "$?" is 0. # If not, then "$?" is 1.
Parameter substitution and/or expansion. The following expressions are the complement to the match in expr string operations (see Example 12−9). These particular ones are used mostly in parsing file path names. Variable length / Substring removal ${#var}
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Advanced Bash−Scripting Guide String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array. Exceptions: ◊ ${#*} and ${#@} give the number of positional parameters. ◊ For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array.
Example 9−15. Length of a variable #!/bin/bash # length.sh E_NO_ARGS=65 if [ $# −eq 0 ] # Must have command−line args to demo script. then echo "Invoke this script with one or more command−line arguments." exit $E_NO_ARGS fi var01=abcdEFGH28ij echo "var01 = ${var01}" echo "Length of var01 = ${#var01}" echo "Number of command−line arguments passed to script = ${#@}" echo "Number of command−line arguments passed to script = ${#*}" exit 0
${var#Pattern}, ${var##Pattern} Remove from $var the shortest/longest part of $Pattern that matches the front end of $var. A usage illustration from Example A−8: # Function from "days−between.sh" example. # Strips leading zero(s) from argument passed. strip_leading_zero () # Strip possible leading zero(s) { #+ from argument passed. return=${1#0} # The "1" refers to "$1" −− passed arg. } # The "0" is what to remove from "$1" −− strips zeros.
Manfred Schwarb's more elaborate variation of the above: strip_leading_zero2 () # Strip possible leading zero(s), since otherwise { # Bash will interpret such numbers as octal values. shopt −s extglob # Turn on extended globbing. local val=${1##+(0)} # Use local variable, longest matching series of 0's. shopt −u extglob # Turn off extended globbing. _strip_leading_zero2=${val:−0} # If input was 0, return 0 instead of "". }
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Advanced Bash−Scripting Guide Another usage illustration: echo `basename $PWD` echo "${PWD##*/}" echo echo `basename $0` echo $0 echo "${0##*/}" echo filename=test.data echo "${filename##*.}"
# Basename of current working directory. # Basename of current working directory. # Name of script. # Name of script. # Name of script.
# data # Extension of filename.
${var%Pattern}, ${var%%Pattern} Remove from $var the shortest/longest part of $Pattern that matches the back end of $var. Version 2 of Bash adds additional options.
Example 9−16. Pattern matching in parameter substitution #!/bin/bash # patt−matching.sh # Pattern matching
using the # ## % %% parameter substitution operators.
var1=abcd12345abc6789 pattern1=a*c # * (wild card) matches everything between a − c. echo echo "var1 = $var1" echo "var1 = ${var1}"
# abcd12345abc6789 # abcd12345abc6789 # (alternate form) echo "Number of characters in ${var1} = ${#var1}" echo echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c') echo "−−−−−−−−−−−−−−" echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789 # Shortest possible match, strips out first 3 characters abcd12345abc6789 # ^^^^^ |−| echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789 # Longest possible match, strips out first 12 characters abcd12345abc6789 # ^^^^^ |−−−−−−−−−−| echo; echo; echo pattern2=b*9 # everything between 'b' and '9' echo "var1 = $var1" # Still abcd12345abc6789 echo echo "pattern2 = $pattern2" echo "−−−−−−−−−−−−−−" echo '${var1%pattern2} =' "${var1%$pattern2}" # # Shortest possible match, strips out last 6 characters # ^^^^ echo '${var1%%pattern2} =' "${var1%%$pattern2}" # # Longest possible match, strips out last 12 characters # ^^^^
abcd12345a abcd12345abc6789 |−−−−| a abcd12345abc6789 |−−−−−−−−−−−−−|
# Remember, # and ## work from the left end (beginning) of string, # % and %% work from the right end.
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Advanced Bash−Scripting Guide echo exit 0
Example 9−17. Renaming file extensions: #!/bin/bash # #
rfe −−−
# Renaming file extensions. # # rfe old_extension new_extension # # Example: # To rename all *.gif files in working directory to *.jpg, # rfe gif jpg ARGS=2 E_BADARGS=65 if [ $# −ne "$ARGS" ] then echo "Usage: `basename $0` old_file_suffix new_file_suffix" exit $E_BADARGS fi for filename in *.$1 # Traverse list of files ending with 1st argument. do mv $filename ${filename%$1}$2 # Strip off part of filename matching 1st argument, #+ then append 2nd argument. done exit 0
Variable expansion / Substring replacement These constructs have been adopted from ksh. ${var:pos} Variable var expanded, starting from offset pos. ${var:pos:len} Expansion to a max of len characters of variable var, from offset pos. See Example A−15 for an example of the creative use of this operator. ${var/Pattern/Replacement} First match of Pattern, within var replaced with Replacement. If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted. ${var//Pattern/Replacement} Global replacement. All matches of Pattern, within var replaced with Replacement. As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted. Chapter 9. Variables Revisited
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Advanced Bash−Scripting Guide Example 9−18. Using pattern matching to parse arbitrary strings #!/bin/bash var1=abcd−1234−defg echo "var1 = $var1" t=${var1#*−*} echo "var1 (with everything, up to and including first − stripped out) = $t" # t=${var1#*−} works just the same, #+ since # matches the shortest string, #+ and * matches everything preceding, including an empty string. # (Thanks, S. C. for pointing this out.) t=${var1##*−*} echo "If var1 contains a \"−\", returns empty string...
var1 = $t"
t=${var1%*−*} echo "var1 (with everything from the last − on stripped out) = $t" echo # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− path_name=/home/bozo/ideas/thoughts.for.today # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− echo "path_name = $path_name" t=${path_name##/*/} echo "path_name, stripped of prefixes = $t" # Same effect as t=`basename $path_name` in this particular case. # t=${path_name%/}; t=${t##*/} is a more general solution, #+ but still fails sometimes. # If $path_name ends with a newline, then `basename $path_name` will not work, #+ but the above expression will. # (Thanks, S.C.) t=${path_name%/*.*} # Same effect as t=`dirname $path_name` echo "path_name, stripped of suffixes = $t" # These will fail in some cases, such as "../", "/foo////", # "foo/", "/". # Removing suffixes, especially when the basename has no suffix, #+ but the dirname does, also complicates matters. # (Thanks, S.C.) echo t=${path_name:11} echo "$path_name, with first 11 chars stripped off = $t" t=${path_name:11:5} echo "$path_name, with first 11 chars stripped off, length 5 = $t" echo t=${path_name/bozo/clown} echo "$path_name with \"bozo\" replaced by \"clown\" = $t" t=${path_name/today/} echo "$path_name with \"today\" deleted = $t" t=${path_name//o/O} echo "$path_name with all o's capitalized = $t" t=${path_name//o/} echo "$path_name with all o's deleted = $t"
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Advanced Bash−Scripting Guide exit 0
${var/#Pattern/Replacement} If prefix of var matches Pattern, then substitute Replacement for Pattern. ${var/%Pattern/Replacement} If suffix of var matches Pattern, then substitute Replacement for Pattern.
Example 9−19. Matching patterns at prefix or suffix of string #!/bin/bash # Pattern replacement at prefix / suffix of string. v0=abc1234zip1234abc echo "v0 = $v0" echo
# Original variable. # abc1234zip1234abc
# Match at prefix (beginning) of string. v1=${v0/#abc/ABCDEF} # abc1234zip1234abc # |−| echo "v1 = $v1" # ABCDE1234zip1234abc # |−−−| # Match at suffix (end) of string. v2=${v0/%abc/ABCDEF} # abc1234zip123abc # |−| echo "v2 = $v2" # abc1234zip1234ABCDEF # |−−−−| echo # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Must match at beginning / end of string, #+ otherwise no replacement results. # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− v3=${v0/#123/000} # Matches, but not at beginning. echo "v3 = $v3" # abc1234zip1234abc # NO REPLACEMENT. v4=${v0/%123/000} # Matches, but not at end. echo "v4 = $v4" # abc1234zip1234abc # NO REPLACEMENT. exit 0
${!varprefix*}, ${!varprefix@} Matches all previously declared variables beginning with varprefix. xyz23=whatever xyz24= a=${!xyz*} echo "a = $a" a=${!xyz@} echo "a = $a"
# # # #
Expands to names of declared variables beginning with "xyz". a = xyz23 xyz24 Same as above. a = xyz23 xyz24
# Bash, version 2.04, adds this feature.
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9.4. Typing variables: declare or typeset The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts. declare/typeset options −r readonly declare −r var1
(declare −r var1 works the same as readonly var1) This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message. −i integer declare −i number # The script will treat subsequent occurrences of "number" as an integer. number=3 echo "Number = $number"
# Number = 3
number=three echo "Number = $number" # Number = 0 # Tries to evaluate the string "three" as an integer.
Certain arithmetic operations are permitted for declared integer variables without the need for expr or let. n=6/3 echo "n = $n"
# n = 6/3
declare −i n n=6/3 echo "n = $n"
# n = 2
−a array declare −a indices
The variable indices will be treated as an array. −f functions declare −f
A declare −f line with no arguments in a script causes a listing of all the functions previously defined in that script. declare −f function_name
A declare −f function_name in a script lists just the function named. Chapter 9. Variables Revisited
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Advanced Bash−Scripting Guide −x export declare −x var3
This declares a variable as available for exporting outside the environment of the script itself. −x var=$value declare −x var3=373
The declare command permits assigning a value to a variable in the same statement as setting its properties.
Example 9−20. Using declare to type variables #!/bin/bash func1 () { echo This is a function. } declare −f
# Lists the function above.
echo declare −i var1 # var1 is an integer. var1=2367 echo "var1 declared as $var1" var1=var1+1 # Integer declaration eliminates the need for 'let'. echo "var1 incremented by 1 is $var1." # Attempt to change variable declared as integer echo "Attempting to change var1 to floating point value, 2367.1." var1=2367.1 # Results in error message, with no change to variable. echo "var1 is still $var1" echo declare −r var2=13.36
# 'declare' permits setting a variable property #+ and simultaneously assigning it a value. echo "var2 declared as $var2" # Attempt to change readonly variable. var2=13.37 # Generates error message, and exit from script. echo "var2 is still $var2"
# This line will not execute.
exit 0
# Script will not exit here.
9.5. Indirect References to Variables Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation.
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Advanced Bash−Scripting Guide Example 9−21. Indirect References #!/bin/bash # Indirect variable referencing. a=letter_of_alphabet letter_of_alphabet=z echo # Direct reference. echo "a = $a" # Indirect reference. eval a=\$$a echo "Now a = $a" echo
# Now, let's try changing the second order reference. t=table_cell_3 table_cell_3=24 echo "\"table_cell_3\" = $table_cell_3" echo −n "dereferenced \"t\" = "; eval echo \$$t # In this simple case, # eval t=\$$t; echo "\"t\" = $t" # also works (why?). echo t=table_cell_3 NEW_VAL=387 table_cell_3=$NEW_VAL echo "Changing value of \"table_cell_3\" to $NEW_VAL." echo "\"table_cell_3\" now $table_cell_3" echo −n "dereferenced \"t\" now "; eval echo \$$t # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3) echo # (Thanks, S.C., for clearing up the above behavior.)
# Another method is the ${!t} notation, discussed in "Bash, version 2" section. # See also example "ex78.sh". exit 0
Example 9−22. Passing an indirect reference to awk #!/bin/bash # Another version of the "column totaler" script #+ that adds up a specified column (of numbers) in the target file. # This uses indirect references. ARGS=2 E_WRONGARGS=65
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Advanced Bash−Scripting Guide if [ $# −ne "$ARGS" ] # Check for proper no. of command line args. then echo "Usage: `basename $0` filename column−number" exit $E_WRONGARGS fi filename=$1 column_number=$2 #===== Same as original script, up to this point =====#
# A multi−line awk script is invoked by
awk ' ..... '
# Begin awk script. # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− awk " { total += \$${column_number} # indirect reference } END { print total } " "$filename" # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # End awk script. # Indirect variable reference avoids the hassles #+ of referencing a shell variable within the embedded awk script. # Thanks, Stephane Chazelas.
exit 0
This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example 35−2) makes indirect referencing more intuitive.
9.6. $RANDOM: generate random integer $RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 − 32767. $RANDOM should not be used to generate an encryption key.
Example 9−23. Generating random numbers #!/bin/bash # $RANDOM returns a different random integer at each invocation. # Nominal range: 0 − 32767 (signed 16−bit integer). MAXCOUNT=10 count=1
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Advanced Bash−Scripting Guide echo echo "$MAXCOUNT random numbers:" echo "−−−−−−−−−−−−−−−−−" while [ "$count" −le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integers. do number=$RANDOM echo $number let "count += 1" # Increment count. done echo "−−−−−−−−−−−−−−−−−" # If you need a random int within a certain range, use the 'modulo' operator. # This returns the remainder of a division operation. RANGE=500 echo number=$RANDOM let "number %= $RANGE" echo "Random number less than $RANGE
−−−
$number"
echo # If you need a random int greater than a lower bound, # then set up a test to discard all numbers below that. FLOOR=200 number=0 #initialize while [ "$number" −le $FLOOR ] do number=$RANDOM done echo "Random number greater than $FLOOR −−− echo
$number"
# May combine above two techniques to retrieve random number between two limits. number=0 #initialize while [ "$number" −le $FLOOR ] do number=$RANDOM let "number %= $RANGE" # Scales $number down within $RANGE. done echo "Random number between $FLOOR and $RANGE −−− $number" echo
# Generate binary choice, that is, "true" or "false" value. BINARY=2 number=$RANDOM T=1 let "number %= $BINARY" # Note that let "number >>= 14" gives a better random distribution #+ (right shifts out everything except last binary digit). if [ "$number" −eq $T ] then echo "TRUE" else echo "FALSE"
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Advanced Bash−Scripting Guide fi echo
# Generate toss of the dice. SPOTS=6 # Modulo 6 gives range 0 − 5. # Incrementing by 1 gives desired range of 1 − 6. # Thanks, Paulo Marcel Coelho Aragao, for the simplification. ZERO=0 die1=0 die2=0 # Tosses each die separately, and so gives correct odds. let "die1 = $RANDOM % $SPOTS +1" # Roll first one. let "die2 = $RANDOM % $SPOTS +1" # Roll second one. let "throw = $die1 + $die2" echo "Throw of the dice = $throw" echo
exit 0
Example 9−24. Picking a random card from a deck #!/bin/bash # pick−card.sh # This is an example of choosing a random element of an array.
# Pick a card, any card. Suites="Clubs Diamonds Hearts Spades" Denominations="2 3 4 5 6 7 8 9 10 Jack Queen King Ace" suite=($Suites) denomination=($Denominations)
# Read into array variable.
num_suites=${#suite[*]} # Count how many elements. num_denominations=${#denomination[*]}
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Advanced Bash−Scripting Guide echo −n "${denomination[$((RANDOM%num_denominations))]} of " echo ${suite[$((RANDOM%num_suites))]}
# $bozo sh pick−cards.sh # Jack of Clubs
# Thank you, "jipe," for pointing out this use of $RANDOM. exit 0
Jipe points out a set of techniques for generating random numbers within a range. # Generate random number between 6 and 30. rnumber=$((RANDOM%25+6)) # Generate random number in the same 6 − 30 range, #+ but the number must be evenly divisible by 3. rnumber=$(((RANDOM%30/3+1)*3)) # Note that this will not work all the time. # It fails if $RANDOM returns 0. #
Exercise: Try to figure out the pattern here.
Bill Gradwohl came up with an improved formula that works for positive numbers. rnumber=$(((RANDOM%(max−min+divisibleBy))/divisibleBy*divisibleBy+min))
Here Bill presents a versatile function that returns a random number between two specified values.
Example 9−25. Random between values #!/bin/bash # random−between.sh # Random number between two specified values. # Script by Bill Gradwohl, with minor modifications by the document author. # Used with permission.
randomBetween() { # Generates a positive or negative random number #+ between $min and $max #+ and divisible by $divisibleBy. # Gives a "reasonably random" distribution of return values. # # Bill Gradwohl − Oct 1, 2003 syntax() { # Function echo echo echo echo echo echo echo echo
embedded within function. "Syntax: randomBetween [min] [max] [multiple]" "Expects up to 3 passed parameters, but all are completely optional." "min is the minimum value" "max is the maximum value" "multiple specifies that the answer must be a multiple of this value." " i.e. answer must be evenly divisible by this number."
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Advanced Bash−Scripting Guide echo echo echo echo echo echo
"If any value is missing, defaults area supplied as: 0 32767 1" "Successful completion returns 0, unsuccessful completion returns" "function syntax and 1." "The answer is returned in the global variable randomBetweenAnswer" "Negative values for any passed parameter are handled correctly."
} local min=${1:−0} local max=${2:−32767} local divisibleBy=${3:−1} # Default values assigned, in case parameters not passed to function. local x local spread # Let's make sure the divisibleBy value is positive. [ ${divisibleBy} −lt 0 ] && divisibleBy=$((0−divisibleBy)) # Sanity check. if [ $# −gt 3 −o ${divisibleBy} −eq 0 −o syntax return 1 fi
${min} −eq ${max} ]; then
# See if the min and max are reversed. if [ ${min} −gt ${max} ]; then # Swap them. x=${min} min=${max} max=${x} fi # If min is itself not evenly divisible by $divisibleBy, #+ then fix the min to be within range. if [ $((min/divisibleBy*divisibleBy)) −ne ${min} ]; then if [ ${min} −lt 0 ]; then min=$((min/divisibleBy*divisibleBy)) else min=$((((min/divisibleBy)+1)*divisibleBy)) fi fi # If max is itself not evenly divisible by $divisibleBy, #+ then fix the max to be within range. if [ $((max/divisibleBy*divisibleBy)) −ne ${max} ]; then if [ ${max} −lt 0 ]; then max=$((((max/divisibleBy)−1)*divisibleBy)) else max=$((max/divisibleBy*divisibleBy)) fi fi # #
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Now do the real work.
# Note that to get a proper distribution for the end points, the #+ range of random values has to be allowed to go between 0 and #+ abs(max−min)+divisibleBy, not just abs(max−min)+1. # The slight increase will produce the proper distribution for the #+ end points.
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Advanced Bash−Scripting Guide # #+ #+ #+ #
Changing the formula to use abs(max−min)+1 will still produce correct answers, but the randomness of those answers is faulty in that the number of times the end points ($min and $max) are returned is considerably lower than when the correct formula is used. −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
spread=$((max−min)) [ ${spread} −lt 0 ] && spread=$((0−spread)) let spread+=divisibleBy randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min)) return 0 # #+ #+ # # #
However, Paulo Marcel Coelho Aragao points out that when $max and $min are not divisible by $divisibleBy, the formula fails. He suggests instead the following formula: rnumber = $(((RANDOM%(max−min+1)+min)/divisibleBy*divisibleBy))
} # Let's test the function. min=−14 max=20 divisibleBy=3
# Generate an array of expected answers and check to make sure we get #+ at least one of each answer if we loop long enough. declare −a answer minimum=${min} maximum=${max} if [ $((minimum/divisibleBy*divisibleBy)) −ne ${minimum} ]; then if [ ${minimum} −lt 0 ]; then minimum=$((minimum/divisibleBy*divisibleBy)) else minimum=$((((minimum/divisibleBy)+1)*divisibleBy)) fi fi
# If max is itself not evenly divisible by $divisibleBy, #+ then fix the max to be within range. if [ $((maximum/divisibleBy*divisibleBy)) −ne ${maximum} ]; then if [ ${maximum} −lt 0 ]; then maximum=$((((maximum/divisibleBy)−1)*divisibleBy)) else maximum=$((maximum/divisibleBy*divisibleBy)) fi fi
# We need to generate only positive array subscripts, #+ so we need a displacement that that will guarantee #+ positive results. displacement=$((0−minimum)) for ((i=${minimum}; i "$OUTFILE" echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−" >> "$OUTFILE" for file in "$( find $directory −type l )" do echo "$file" done | sort >> "$OUTFILE" # ^^^^^^^^^^^^^
# −type l = symbolic links
# stdout of loop redirected to save file.
exit 0
There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.
Example 10−12. A C−like for loop #!/bin/bash # Two ways to count up to 10. echo # Standard syntax. for a in 1 2 3 4 5 6 7 8 9 10 do echo −n "$a " done echo; echo # +==========================================+
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Advanced Bash−Scripting Guide # Now, let's do the same, using C−like syntax. LIMIT=10 for ((a=1; a $IMAGE_DIRECTORY/$CONTENTSFILE # The "l" option gives a "long" file listing. # The "R" option makes the listing recursive. # The "F" option marks the file types (directories get a trailing /). echo "Creating table of contents." # Create an image file preparatory to burning it onto the CDR.
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Advanced Bash−Scripting Guide mkisofs −r −o $IMAGEFILE $IMAGE_DIRECTORY echo "Creating ISO9660 file system image ($IMAGEFILE)." # Burn the CDR. cdrecord −v −isosize speed=$SPEED dev=0,0 $IMAGEFILE echo "Burning the disk." echo "Please be patient, this will take a while." exit 0
cat, tac cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files. # Uses of 'cat' cat filename
# Lists the file.
cat file.1 file.2 file.3 > file.123
# Combines three files into one.
The −n option to cat inserts consecutive numbers before all lines of the target file(s). The −b option numbers only the non−blank lines. The −v option echoes nonprintable characters, using ^ notation. The −s option squeezes multiple consecutive blank lines into a single blank line. See also Example 12−24 and Example 12−20. In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat the file. cat filename | tr a−z A−Z tr a−z A−Z < filename
# Same effect, but starts one less process, #+ and also dispenses with the pipe.
tac, is the inverse of cat, listing a file backwards from its end. rev reverses each line of a file, and outputs to stdout. This is not the same effect as tac, as it preserves the order of the lines, but flips each one around. bash$ cat file1.txt This is line 1. This is line 2.
bash$ tac file1.txt This is line 2. This is line 1.
bash$ rev file1.txt .1 enil si sihT .2 enil si sihT
cp This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see Example 12−6).
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Advanced Bash−Scripting Guide Particularly useful are the −a archive flag (for copying an entire directory tree) and the −r and −R recursive flags. mv This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see Example 9−17 and Example A−3. When used in a non−interactive script, mv takes the −f (force) option to bypass user input. When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory. bash$ mv source_directory target_directory bash$ ls −lF target_directory total 1 drwxrwxr−x 2 bozo bozo
1024 May 28 19:20 source_directory/
rm Delete (remove) a file or files. The −f option forces removal of even readonly files, and is useful for bypassing user input in a script. The rm command will, by itself, fail to remove filenames beginning with a dash. bash$ rm −badname rm: invalid option −− b Try `rm −−help' for more information.
The way to accomplish this is to preface the filename to be removed with a dot−slash . bash$ rm ./−badname
When used with the recursive flag −r, this command removes files all the way down the directory tree from the current directory. rmdir Remove directory. The directory must be empty of all files, including invisible "dotfiles", [28] for this command to succeed. mkdir Make directory, creates a new directory. For example, mkdir −p project/programs/December creates the named directory. The −p option automatically creates any necessary parent directories. chmod Changes the attributes of an existing file (see Example 11−11). chmod +x filename # Makes "filename" executable for all users. chmod u+s filename # Sets "suid" bit on "filename" permissions. # An ordinary user may execute "filename" with same privileges as the file's owner.
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Advanced Bash−Scripting Guide # (This does not apply to shell scripts.) chmod 644 filename # Makes "filename" readable/writable to owner, readable to # others # (octal mode). chmod 1777 directory−name # Gives everyone read, write, and execute permission in directory, # however also sets the "sticky bit". # This means that only the owner of the directory, # owner of the file, and, of course, root # can delete any particular file in that directory.
chattr Change file attributes. This has the same effect as chmod above, but with a different invocation syntax, and it works only on an ext2 filesystem. ln Creates links to pre−existings files. A "link" is a reference to a file, an alternate name for it. The ln command permits referencing the linked file by more than one name and is a superior alternative to aliasing (see Example 4−6). The ln creates only a reference, a pointer to the file only a few bytes in size. The ln command is most often used with the −s, symbolic or "soft" link flag. An advantage of using the −s flag is that it permits linking across file systems. The syntax of the command is a bit tricky. For example: ln −s oldfile newfile links the previously existing oldfile to the newly created link, newfile. If a file named newfile has previously existed, it will be deleted when the filename newfile is preempted as the name for a link.
Which type of link to use? As John Macdonald explains it: Both of these provide a certain measure of dual reference −− if you edit the contents of the file using any name, your changes will affect both the original name and either a hard or soft new name. The differences between them occurs when you work at a higher level. The advantage of a hard link is that the new name is totally independent of the old name −− if you remove or rename the old name, that does not affect the hard link, which continues to point to the data while it would leave a soft link hanging pointing to the old name which is no longer there. The advantage of a soft link is that it can refer to a different file system (since it is just a reference to a file name, not to actual data). Links give the ability to invoke a script (or any other type of executable) with multiple names, and having that script behave according to how it was invoked.
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Advanced Bash−Scripting Guide #!/bin/bash # hello.sh: Saying "hello" or "goodbye" #+ depending on how script is invoked. # # # # #
Make a link in current working directory ($PWD) to this script: ln −s hello.sh goodbye Now, try invoking this script both ways: ./hello.sh ./goodbye
HELLO_CALL=65 GOODBYE_CALL=66 if [ $0 = "./goodbye" ] then echo "Good−bye!" # Some other goodbye−type commands, as appropriate. exit $GOODBYE_CALL fi echo "Hello!" # Some other hello−type commands, as appropriate. exit $HELLO_CALL
man, info These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain a more detailed description than do the man pages.
12.2. Complex Commands Commands for more advanced users find −exec COMMAND \; Carries out COMMAND on each file that find matches. The command sequence terminates with \; (the ";" is escaped to make certain the shell passes it to find literally). bash$ find ~/ −name '*.txt' /home/bozo/.kde/share/apps/karm/karmdata.txt /home/bozo/misc/irmeyc.txt /home/bozo/test−scripts/1.txt
If COMMAND contains {}, then find substitutes the full path name of the selected file for "{}". find ~/ −name 'core*' −exec rm {} \; # Removes all core dump files from user's home directory. find /home/bozo/projects −mtime 1 # Lists all files in /home/bozo/projects directory tree #+ that were modified within the last day. # # mtime = last modification time of the target file
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Advanced Bash−Scripting Guide # #
ctime = last status change time (via 'chmod' or otherwise) atime = last access time
DIR=/home/bozo/junk_files find "$DIR" −type f −atime +5 −exec rm {} \; # ^^ # Curly brackets are placeholder for the pathname output by "find." # # Deletes all files in "/home/bozo/junk_files" #+ that have not been accessed in at least 5 days. # # "−type filetype", where # f = regular file # d = directory, etc. # (The 'find' manpage has a complete listing.) find /etc −exec grep '[0−9][0−9]*[.][0−9][0−9]*[.][0−9][0−9]*[.][0−9][0−9]*' {} \; # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. # There a few extraneous hits − how can they be filtered out? # Perhaps by: find /etc −type f −exec cat '{}' \; | tr −c '.[:digit:]' '\n' \ | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' # [:digit:] is one of the character classes # introduced with the POSIX 1003.2 standard. # Thanks, S.C.
The −exec option to find should not be confused with the exec shell builtin.
Example 12−3. Badname, eliminate file names in current directory containing bad characters and whitespace. #!/bin/bash # badname.sh # Delete filenames in current directory containing bad characters. for filename in * do badname=`echo "$filename" | sed −n /[\+\{\;\"\\\=\?~\(\)\\&\*\|\$]/p` # badname=`echo "$filename" | sed −n '/[+{;"\=?~()&*|$]/p'` also works. # Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $ # rm $badname 2>/dev/null # ^^^^^^^^^^^ Error messages deep−sixed. done # Now, take care of files containing all manner of whitespace. find . −name "* *" −exec rm −f {} \; # The path name of the file that "find" finds replaces the "{}". # The '\' ensures that the ';' is interpreted literally, as end of command. exit 0 #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
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Advanced Bash−Scripting Guide # Commands below this line will not execute because of "exit" command. # An alternative to the above script: find . −name '*[+{;"\\=?~()&*|$ ]*' −exec rm −f '{}' \; # (Thanks, S.C.)
Example 12−4. Deleting a file by its inode number #!/bin/bash # idelete.sh: Deleting a file by its inode number. # This is useful when a filename starts with an illegal character, #+ such as ? or −. ARGCOUNT=1 E_WRONGARGS=70 E_FILE_NOT_EXIST=71 E_CHANGED_MIND=72
# Filename arg must be passed to script.
if [ $# −ne "$ARGCOUNT" ] then echo "Usage: `basename $0` filename" exit $E_WRONGARGS fi if [ ! −e "$1" ] then echo "File \""$1"\" does not exist." exit $E_FILE_NOT_EXIST fi inum=`ls −i | grep "$1" | awk '{print $1}'` # inum = inode (index node) number of file # Every file has an inode, a record that hold its physical address info. echo; echo −n "Are you absolutely sure you want to delete \"$1\" (y/n)? " # The '−v' option to 'rm' also asks this. read answer case "$answer" in [nN]) echo "Changed your mind, huh?" exit $E_CHANGED_MIND ;; *) echo "Deleting file \"$1\".";; esac find . −inum $inum −exec rm {} \; # ^^ # Curly brackets are placeholder #+ for text output by "find." echo "File "\"$1"\" deleted!" exit 0
See Example 12−25, Example 3−4, and Example 10−9 for scripts using find. Its manpage provides more detail on this complex and powerful command. xargs A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Chapter 12. External Filters, Programs and Commands
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Advanced Bash−Scripting Guide Consider it as a powerful replacement for backquotes. In situations where backquotes fail with a too many arguments error, substituting xargs often works. Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file. The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out. bash$ ls −l total 0 −rw−rw−r−− −rw−rw−r−−
1 bozo 1 bozo
bozo bozo
0 Jan 29 23:58 file1 0 Jan 29 23:58 file2
bash$ ls −l | xargs total 0 −rw−rw−r−− 1 bozo bozo 0 Jan 29 23:58 file1 −rw−rw−r−− 1 bozo bozo 0 Jan 29 23:58
ls | xargs −p −l gzip gzips every file in current directory, one at a time, prompting before each operation. An interesting xargs option is −n NN, which limits to NN the number of arguments passed. ls | xargs −n 8 echo lists the files in the current directory in 8 columns. Another useful option is −0, in combination with find −print0 or grep −lZ. This allows handling arguments containing whitespace or quotes. find / −type f −print0 | xargs −0 grep −liwZ GUI | xargs −0 rm −f grep −rliwZ GUI / | xargs −0 rm −f Either of the above will remove any file containing "GUI". (Thanks, S.C.)
Example 12−5. Logfile: Using xargs to monitor system log #!/bin/bash # Generates a log file in current directory # from the tail end of /var/log/messages. # Note: /var/log/messages must be world readable # if this script invoked by an ordinary user. # #root chmod 644 /var/log/messages LINES=5 ( date; uname −a ) >>logfile # Time and machine name echo −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− >>logfile tail −$LINES /var/log/messages | xargs | fmt −s >>logfile echo >>logfile echo >>logfile
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Advanced Bash−Scripting Guide exit 0 # Exercise: # −−−−−−−− # Modify this script to track changes in /var/log/messages at intervals #+ of 20 minutes. # Hint: Use the "watch" command.
As in find, a curly bracket pair serves as a placeholder for replacement text.
Example 12−6. Copying files in current directory to another #!/bin/bash # copydir.sh # Copy (verbose) all files in current directory ($PWD) #+ to directory specified on command line. E_NOARGS=65 if [ −z "$1" ] # Exit if no argument given. then echo "Usage: `basename $0` directory−to−copy−to" exit $E_NOARGS fi ls # # # # # # # #+ #+ # # #+ #+
. | xargs −i −t cp ./{} $1 ^^ ^^ ^^ −t is "verbose" (output command line to stderr) option. −i is "replace strings" option. {} is a placeholder for output text. This is similar to the use of a curly bracket pair in "find." List the files in current directory (ls .), pass the output of "ls" as arguments to "xargs" (−i −t options), then copy (cp) these arguments ({}) to new directory ($1). The net result is the exact equivalent of cp * $1 unless any of the filenames has embedded "whitespace" characters.
exit 0
Example 12−7. Killing processes by name #!/bin/bash # kill−byname.sh: Killing processes by name. # Compare this script with kill−process.sh. # For instance, #+ try "./kill−byname.sh xterm" −− #+ and watch all the xterms on your desktop disappear. # # #
Warning: −−−−−−− This is a fairly dangerous script.
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Advanced Bash−Scripting Guide # Running it carelessly (especially as root) #+ can cause data loss and other undesirable effects. E_BADARGS=66 if test −z "$1" # No command line arg supplied? then echo "Usage: `basename $0` Process(es)_to_kill" exit $E_BADARGS fi
PROCESS_NAME="$1" ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs −i kill {} 2&>/dev/null # ^^ ^^ # # # # # #
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Notes: −i is the "replace strings" option to xargs. The curly brackets are the placeholder for the replacement. 2&>/dev/null suppresses unwanted error messages. −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
exit $?
Example 12−8. Word frequency analysis using xargs #!/bin/bash # wf2.sh: Crude word frequency analysis on a text file. # Uses 'xargs' to decompose lines of text into single words. # Compare this example to the "wf.sh" script later on.
# Check for input file on command line. ARGS=1 E_BADARGS=65 E_NOFILE=66 if [ $# −ne "$ARGS" ] # Correct number of arguments passed to script? then echo "Usage: `basename $0` filename" exit $E_BADARGS fi if [ ! −f "$1" ] # Check if file exists. then echo "File \"$1\" does not exist." exit $E_NOFILE fi
######################################################## cat "$1" | xargs −n1 | \ # List the file, one word per line. tr A−Z a−z | \ # Shift characters to lowercase. sed −e 's/\.//g' −e 's/\,//g' −e 's/ /\
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Advanced Bash−Scripting Guide /g' | \ # Filter out periods and commas, and #+ change space between words to linefeed, sort | uniq −c | sort −nr # Finally prefix occurrence count and sort numerically. ######################################################## # This does the same job as the "wf.sh" example, #+ but a bit more ponderously, and it runs more slowly. exit 0
expr All−purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical. expr 3 + 5 returns 8 expr 5 % 3 returns 2 expr 5 \* 3 returns 15 The multiplication operator must be escaped when used in an arithmetic expression with expr. y=`expr $y + 1` Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion. z=`expr substr $string $position $length` Extract substring of $length characters, starting at $position. Example 12−9. Using expr #!/bin/bash # Demonstrating some of the uses of 'expr' # ======================================= echo # Arithmetic Operators # −−−−−−−−−− −−−−−−−−− echo "Arithmetic Operators" echo a=`expr 5 + 3` echo "5 + 3 = $a" a=`expr $a + 1` echo echo "a + 1 = $a" echo "(incrementing a variable)" a=`expr 5 % 3` # modulo echo echo "5 mod 3 = $a"
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Advanced Bash−Scripting Guide echo echo # Logical Operators # −−−−−−− −−−−−−−−− # Returns 1 if true, 0 if false, #+ opposite of normal Bash convention. echo "Logical Operators" echo x=24 y=25 b=`expr $x = $y` echo "b = $b" echo
# Test equality. # 0 ( $x −ne $y )
a=3 b=`expr $a \> 10` echo 'b=`expr $a \> 10`, therefore...' echo "If a > 10, b = 0 (false)" echo "b = $b" # 0 ( 3 ! −gt 10 ) echo b=`expr $a \< 10` echo "If a < 10, b = 1 (true)" echo "b = $b" # 1 ( 3 −lt 10 ) echo # Note escaping of operators. b=`expr $a \ final.list # Concatenates the list files, # sorts them, # removes duplicate lines, # and finally writes the result to an output file.
The useful −c option prefixes each line of the input file with its number of occurrences. bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times.
bash$ uniq −c testfile 1 This line occurs only once. 2 This line occurs twice. 3 This line occurs three times.
bash$ sort testfile | uniq −c | sort −nr 3 This line occurs three times. 2 This line occurs twice. 1 This line occurs only once.
The sort INPUTFILE | uniq −c | sort −nr command string produces a frequency of occurrence listing on the INPUTFILE file (the −nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined.
Example 12−11. Word Frequency Analysis
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Advanced Bash−Scripting Guide #!/bin/bash # wf.sh: Crude word frequency analysis on a text file. # This is a more efficient version of the "wf2.sh" script.
# Check for input file on command line. ARGS=1 E_BADARGS=65 E_NOFILE=66 if [ $# −ne "$ARGS" ] # Correct number of arguments passed to script? then echo "Usage: `basename $0` filename" exit $E_BADARGS fi if [ ! −f "$1" ] # Check if file exists. then echo "File \"$1\" does not exist." exit $E_NOFILE fi
######################################################## # main () sed −e 's/\.//g' −e 's/\,//g' −e 's/ /\ /g' "$1" | tr 'A−Z' 'a−z' | sort | uniq −c | sort −nr # ========================= # Frequency of occurrence # #+ #+ #+
Filter out periods and commas, and change space between words to linefeed, then shift characters to lowercase, and finally prefix occurrence count and sort numerically.
# Arun Giridhar suggests modifying the above to: # . . . | sort | uniq −c | sort +1 [−f] | sort +0 −nr # This adds a secondary sort key, so instances of #+ equal occurrence are sorted alphabetically. # As he explains it: # "This is effectively a radix sort, first on the #+ least significant column #+ (word or string, optionally case−insensitive) #+ and last on the most significant column (frequency)." ######################################################## exit 0 # Exercises: # −−−−−−−−− # 1) Add 'sed' commands to filter out other punctuation, #+ such as semicolons. # 2) Modify to also filter out multiple spaces and other whitespace. bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times.
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bash$ ./wf.sh testfile 6 this 6 occurs 6 line 3 times 3 three 2 twice 1 only 1 once
expand, unexpand The expand filter converts tabs to spaces. It is often used in a pipe. The unexpand filter converts spaces to tabs. This reverses the effect of expand. cut A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the −d (delimiter) and −f (field specifier) options. Using cut to obtain a listing of the mounted filesystems: cat /etc/mtab | cut −d ' ' −f1,2
Using cut to list the OS and kernel version: uname −a | cut −d" " −f1,3,11,12
Using cut to extract message headers from an e−mail folder: bash$ grep '^Subject:' read−messages | cut −c10−80 Re: Linux suitable for mission−critical apps? MAKE MILLIONS WORKING AT HOME!!! Spam complaint Re: Spam complaint
Using cut to parse a file: # List all the users in /etc/passwd. FILENAME=/etc/passwd for user in $(cut −d: −f1 $FILENAME) do echo $user done # Thanks, Oleg Philon for suggesting this.
cut −d ' ' −f2,3 filename is equivalent to awk −F'[ ]' '{ print $2, $3 }' filename See also Example 12−37. paste Chapter 12. External Filters, Programs and Commands
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Advanced Bash−Scripting Guide Tool for merging together different files into a single, multi−column file. In combination with cut, useful for creating system log files. join Consider this a special−purpose cousin of paste. This powerful utility allows merging two files in a meaningful fashion, which essentially creates a simple version of a relational database. The join command operates on exactly two files, but pastes together only those lines with a common tagged field (usually a numerical label), and writes the result to stdout. The files to be joined should be sorted according to the tagged field for the matchups to work properly. File: 1.data 100 Shoes 200 Laces 300 Socks File: 2.data 100 $40.00 200 $1.00 300 $2.00 bash$ join 1.data 2.data File: 1.data 2.data 100 Shoes $40.00 200 Laces $1.00 300 Socks $2.00
The tagged field appears only once in the output. head lists the beginning of a file to stdout (the default is 10 lines, but this can be changed). It has a number of interesting options. Example 12−12. Which files are scripts? #!/bin/bash # script−detector.sh: Detects scripts within a directory. TESTCHARS=2 SHABANG='#!'
# Test first 2 characters. # Scripts begin with a "sha−bang."
for file in * # Traverse all the files in current directory. do if [[ `head −c$TESTCHARS "$file"` = "$SHABANG" ]] # head −c2 #! # The '−c' option to "head" outputs a specified #+ number of characters, rather than lines (the default). then echo "File \"$file\" is a script." else echo "File \"$file\" is *not* a script." fi done
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Advanced Bash−Scripting Guide exit 0
Example 12−13. Generating 10−digit random numbers #!/bin/bash # rnd.sh: Outputs a 10−digit random number # Script by Stephane Chazelas. head −c4 /dev/urandom | od −N4 −tu4 | sed −ne '1s/.* //p'
# =================================================================== # # Analysis # −−−−−−−− # head: # −c4 option takes first 4 bytes. # od: # −N4 option limits output to 4 bytes. # −tu4 option selects unsigned decimal format for output. # sed: # −n option, in combination with "p" flag to the "s" command, # outputs only matched lines.
# The author of this script explains the action of 'sed', as follows. # head −c4 /dev/urandom | od −N4 −tu4 | sed −ne '1s/.* //p' # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−> | # Assume output up to "sed" −−−−−−−−> | # is 0000000 1198195154\n # # # # # # # # # #
sed begins reading characters: 0000000 1198195154\n. Here it finds a newline character, so it is ready to process the first line (0000000 1198195154). It looks at its s. The first and only one is range 1
action s/.* //p
The line number is in the range, so it executes the action: tries to substitute the longest string ending with a space in the line ("0000000 ") with nothing (//), and if it succeeds, prints the result ("p" is a flag to the "s" command here, this is different from the "p" command).
# sed is now ready to continue reading its input. (Note that before # continuing, if −n option had not been passed, sed would have printed # the line once again). # # # #
Now, sed reads the remainder of the characters, and finds the end of the file. It is now ready to process its 2nd line (which is also numbered '$' as it's the last one). It sees it is not matched by any , so its job is done.
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Advanced Bash−Scripting Guide # In few word this sed commmand means: # "On the first line only, remove any character up to the right−most space, # then print it." # A better way to do this would have been: # sed −e 's/.* //;q' # Here, two s (could have been written # sed −e 's/.* //' −e q): # # #
range nothing (matches line) nothing (matches line)
action s/.* // q (quit)
# Here, sed only reads its first line of input. # It performs both actions, and prints the line (substituted) before quitting # (because of the "q" action) since the "−n" option is not passed. # =================================================================== # # A simpler altenative to the above 1−line script would be: # head −c4 /dev/urandom| od −An −tu4 exit 0
See also Example 12−33. tail lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to a system logfile, using the −f option, which outputs lines appended to the file.
Example 12−14. Using tail to monitor the system log #!/bin/bash filename=sys.log cat /dev/null > $filename; echo "Creating / cleaning out file." # Creates file if it does not already exist, #+ and truncates it to zero length if it does. # : > filename and > filename also work. tail /var/log/messages > $filename # /var/log/messages must have world read permission for this to work. echo "$filename contains tail end of system log." exit 0
See also Example 12−5, Example 12−33 and Example 30−6. grep A multi−purpose file search tool that uses Regular Expressions. It was originally a command/filter in the venerable ed line editor: g/re/p −− global − regular expression − print. grep pattern [file...] Search the target file(s) for occurrences of pattern, where pattern may be literal text or a Chapter 12. External Filters, Programs and Commands
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Advanced Bash−Scripting Guide Regular Expression. bash$ grep '[rst]ystem.$' osinfo.txt The GPL governs the distribution of the Linux operating system.
If no target file(s) specified, grep works as a filter on stdout, as in a pipe. bash$ ps ax | grep clock 765 tty1 S 0:00 xclock 901 pts/1 S 0:00 grep clock
The −i option causes a case−insensitive search. The −w option matches only whole words. The −l option lists only the files in which matches were found, but not the matching lines. The −r (recursive) option searches files in the current working directory and all subdirectories below it. The −n option lists the matching lines, together with line numbers. bash$ grep −n Linux osinfo.txt 2:This is a file containing information about Linux. 6:The GPL governs the distribution of the Linux operating system.
The −v (or −−invert−match) option filters out matches. grep pattern1 *.txt | grep −v pattern2 # Matches all lines in "*.txt" files containing "pattern1", # but ***not*** "pattern2".
The −c (−−count) option gives a numerical count of matches, rather than actually listing the matches. grep −c txt *.sgml
# (number of occurrences of "txt" in "*.sgml" files)
# grep −cz . # ^ dot # means count (−c) zero−separated (−z) items matching "." # that is, non−empty ones (containing at least 1 character). # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −cz . printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −cz '$' printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −cz '^' # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −c '$' # By default, newline chars (\n) separate items to match.
# 4 # 5 # 5 # 9
# Note that the −z option is GNU "grep" specific.
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Advanced Bash−Scripting Guide # Thanks, S.C.
When invoked with more than one target file given, grep specifies which file contains matches. bash$ grep Linux osinfo.txt misc.txt osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system. misc.txt:The Linux operating system is steadily gaining in popularity.
To force grep to show the filename when searching only one target file, simply give /dev/null as the second file. bash$ grep Linux osinfo.txt /dev/null osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system.
If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the −q option to suppress output. SUCCESS=0 word=Linux filename=data.file
# if grep lookup succeeds
grep −q "$word" "$filename"
# The "−q" option causes nothing to echo to stdout.
if [ $? −eq $SUCCESS ] # if grep −q "$word" "$filename" can replace lines 5 − 7. then echo "$word found in $filename" else echo "$word not found in $filename" fi
Example 30−6 demonstrates how to use grep to search for a word pattern in a system logfile.
Example 12−15. Emulating "grep" in a script #!/bin/bash # grp.sh: Very crude reimplementation of 'grep'. E_BADARGS=65 if [ −z "$1" ] # Check for argument to script. then echo "Usage: `basename $0` pattern" exit $E_BADARGS fi echo for file in * # Traverse all files in $PWD. do output=$(sed −n /"$1"/p $file) # Command substitution.
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Advanced Bash−Scripting Guide if [ ! −z "$output" ] # What happens if "$output" is not quoted? then echo −n "$file: " echo $output fi # sed −ne "/$1/s|^|${file}: |p" is equivalent to above. echo done echo exit 0 # # # #
Exercises: −−−−−−−−− 1) Add newlines to output, if more than one match in any given file. 2) Add features.
How can grep search for two separate patterns? What if you want grep to display all lines in a file or files that contain both "pattern1" and "pattern2"? One method of accomplishing this is to pipe the result of grep pattern1 to grep pattern2. # tstfile This This This This Here
is a sample file. is an ordinary text file. file does not contain any unusual text. file is not unusual. is some text.
bash$ grep file tstfile # tstfile This is a sample file. This is an ordinary text file. This file does not contain any unusual text. This file is not unusual. bash$ grep file tstfile | grep text This is an ordinary text file. This file does not contain any unusual text.
egrep (extended grep) is the same as grep −E. This uses a somewhat different, extended set of Regular Expressions, which can make the search a bit more flexible. fgrep (fast grep) is the same as grep −F. It does a literal string search (no regular expressions), which allegedly speeds things up a bit. agrep (approximate grep) extends the capabilities of grep to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution. To search compressed files, use zgrep, zegrep, or zfgrep. These also work on non−compressed files, though slower than plain grep, egrep, fgrep. They are handy for searching through a mixed set of files, some compressed, some not. Chapter 12. External Filters, Programs and Commands
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Advanced Bash−Scripting Guide To search bzipped files, use bzgrep. look The command look works like grep, but does a lookup on a "dictionary", a sorted word list. By default, look searches for a match in /usr/dict/words, but a different dictionary file may be specified.
Example 12−16. Checking words in a list for validity #!/bin/bash # lookup: Does a dictionary lookup on each word in a data file. file=words.data
# Data file from which to read words to test.
echo while [ "$word" != end ] # Last word in data file. do read word # From data file, because of redirection at end of loop. look $word > /dev/null # Don't want to display lines in dictionary file. lookup=$? # Exit status of 'look' command. if [ "$lookup" −eq 0 ] then echo "\"$word\" is valid." else echo "\"$word\" is invalid." fi done /dev/null then echo "\"$word\" is valid." else echo "\"$word\" is invalid." fi done /dev/null if [ $? −eq 0 ] # Test exit status of "cmp" command. then echo "File \"$1\" is identical to file \"$2\"." else echo "File \"$1\" differs from file \"$2\"." fi exit 0
Use zcmp on gzipped files. comm Versatile file comparison utility. The files must be sorted for this to be useful. comm −options first−file second−file comm file−1 file−2 outputs three columns: ◊ column 1 = lines unique to file−1 ◊ column 2 = lines unique to file−2 ◊ column 3 = lines common to both. The options allow suppressing output of one or more columns. ◊ −1 suppresses column 1 ◊ −2 suppresses column 2 ◊ −3 suppresses column 3 ◊ −12 suppresses both columns 1 and 2, etc. Chapter 12. External Filters, Programs and Commands
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Advanced Bash−Scripting Guide Utilities basename Strips the path information from a file name, printing only the file name. The construction basename $0 lets the script know its name, that is, the name it was invoked by. This can be used for "usage" messages if, for example a script is called with missing arguments: echo "Usage: `basename $0` arg1 arg2 ... argn"
dirname Strips the basename from a filename, printing only the path information. basename and dirname can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see Example A−8).
Example 12−31. basename and dirname #!/bin/bash a=/home/bozo/daily−journal.txt echo echo echo echo echo
"Basename of /home/bozo/daily−journal.txt = `basename $a`" "Dirname of /home/bozo/daily−journal.txt = `dirname $a`" "My own home is `basename ~/`." "The home of my home is `dirname ~/`."
# Also works with just ~. # Also works with just ~.
exit 0
split Utility for splitting a file into smaller chunks. Usually used for splitting up large files in order to back them up on floppies or preparatory to e−mailing or uploading them. sum, cksum, md5sum These are utilities for generating checksums. A checksum is a number mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. For security applications, use the 128−bit md5sum (message digest checksum) command. bash$ cksum /boot/vmlinuz 1670054224 804083 /boot/vmlinuz
bash$ md5sum /boot/vmlinuz 0f43eccea8f09e0a0b2b5cf1dcf333ba
/boot/vmlinuz
Note that cksum also shows the size, in bytes, of the target file.
Example 12−32. Checking file integrity #!/bin/bash # file−integrity.sh: Checking whether files in a given directory
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Advanced Bash−Scripting Guide #
have been tampered with.
E_DIR_NOMATCH=70 E_BAD_DBFILE=71 dbfile=File_record.md5 # Filename for storing records (database file).
set_up_database () { echo ""$directory"" > "$dbfile" # Write directory name to first line of file. md5sum "$directory"/* >> "$dbfile" # Append md5 checksums and filenames. } check_database () { local n=0 local filename local checksum # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # # This file check should be unnecessary, #+ but better safe than sorry. if [ ! −r "$dbfile" ] then echo "Unable to read checksum database file!" exit $E_BAD_DBFILE fi # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # while read record[n] do directory_checked="${record[0]}" if [ "$directory_checked" != "$directory" ] then echo "Directories do not match up!" # Tried to use file for a different directory. exit $E_DIR_NOMATCH fi if [ "$n" −gt 0 ] # Not directory name. then filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' ) # md5sum writes records backwards, #+ checksum first, then filename. checksum[n]=$( md5sum "${filename[n]}" )
if [ "${record[n]}" = "${checksum[n]}" ] then echo "${filename[n]} unchanged." elif [ "`basename ${filename[n]}`" != "$dbfile" ] # Skip over checksum database file, #+ as it will change with each invocation of script. # −−− # This unfortunately means that when running
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Advanced Bash−Scripting Guide #+ this script on $PWD, tampering with the #+ checksum database file will not be detected. # Exercise: Fix this. then echo "${filename[n]} : CHECKSUM ERROR!" # File has been changed since last checked. fi fi
let "n+=1" done 0; x−−)) do bot=$(echo "scale=9; $interest_rate^$x" | bc) bottom=$(echo "scale=9; $bottom+$bot" | bc) # bottom = $(($bottom + $bot")) done # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Rick Boivie pointed out a more efficient implementation #+ of the above loop, which decreases computation time by 2/3. # for ((x=1; x .]ds.xd1 /dev/null) # on non−GNU systems
echo echo "Key pressed was \""$Keypress"\"." echo stty "$old_tty_settings"
# Restore old settings.
# Thanks, Stephane Chazelas. exit 0
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Advanced Bash−Scripting Guide Also see Example 9−3.
terminals and modes Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does not immediately go to the program actually running in this terminal. A buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program running. There is even a basic line editor inside the terminal. bash$ stty −a speed 9600 baud; rows 36; columns 96; line = 0; intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = ; eol2 = ; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; ... isig icanon iexten echo echoe echok −echonl −noflsh −xcase −tostop −echoprt
Using canonical mode, it is possible to redefine the special keys for the local terminal line editor. bash$ cat > filexxx whaIfoo barhello world bash$ cat filexxx hello world bash$ bash$ wc −c < file 13
The process controlling the terminal receives only 13 characters (12 alphabetic ones, plus a newline), although the user hit 26 keys. In non−canonical ("raw") mode, every key hit (including special editing keys such as ctl−H) sends a character immediately to the controlling process. The Bash prompt disables both icanon and echo, since it replaces the basic terminal line editor with its own more elaborate one. For example, when you hit ctl−A at the Bash prompt, there's no ^A echoed by the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line. Stephane Chazelas setterm Set certain terminal attributes. This command writes to its terminal's stdout a string that changes the behavior of that terminal. bash$ setterm −cursor off bash$
The setterm command can be used within a script to change the appearance of text written to stdout, although there are certainly better tools available for this purpose. setterm −bold on echo bold hello
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Advanced Bash−Scripting Guide setterm −bold off echo normal hello
tset Show or initialize terminal settings. This is a less capable version of stty. bash$ tset −r Terminal type is xterm−xfree86. Kill is control−U (^U). Interrupt is control−C (^C).
setserial Set or display serial port parameters. This command must be run by root user and is usually found in a system setup script. # From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed −e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ
getty, agetty The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty. mesg Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal. It can be very annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi−user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions. wall This is an acronym for "write all", i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 17−2). bash$ wall System going down for maintenance in 5 minutes! Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001... System going down for maintenance in 5 minutes!
If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to it. dmesg Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script. bash$ dmesg | grep hda Kernel command line: ro root=/dev/hda2 hda: IBM−DLGA−23080, ATA DISK drive hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63 hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4
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Advanced Bash−Scripting Guide Information and Statistics uname Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the −a option, gives verbose system info (see Example 12−5). The −s option shows only the OS type. bash$ uname −a Linux localhost.localdomain 2.2.15−2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown bash$ uname −s Linux
arch Show system architecture. Equivalent to uname −m. See Example 10−26. bash$ arch i686 bash$ uname −m i686
lastcomm Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities. lastlog List the last login time of all system users. This references the /var/log/lastlog file. bash$ lastlog root tty1 bin daemon ... bozo tty1
Fri Dec 7 18:43:21 −0700 2001 **Never logged in** **Never logged in** Sat Dec
bash$ lastlog | grep root root tty1
Fri Dec
8 21:14:29 −0700 2001
7 18:43:21 −0700 2001
This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file. lsof List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results. bash$ lsof COMMAND PID init 1 init 1 init 1 cardmgr 213 ...
USER root root root root
FD mem mem mem mem
TYPE REG REG REG REG
DEVICE 3,5 3,5 3,5 3,5
SIZE 30748 73120 931668 36956
NODE NAME 30303 /sbin/init 8069 /lib/ld−2.1.3.so 8075 /lib/libc−2.1.3.so 30357 /sbin/cardmgr
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Advanced Bash−Scripting Guide Diagnostic and debugging tool for tracing system calls and signals. The simplest way of invoking it is strace COMMAND. bash$ strace df execve("/bin/df", ["df"], [/* 45 vars */]) = 0 uname({sys="Linux", node="bozo.localdomain", ...}) = 0 brk(0) = 0x804f5e4 ...
This is the Linux equivalent of truss. nmap Network port scanner. This command scans a server to locate open ports and the services associated with those ports. It is an important security tool for locking down a network against hacking attempts. #!/bin/bash SERVER=$HOST PORT_NUMBER=25
# localhost.localdomain (127.0.0.1). # SMTP port.
nmap $SERVER | grep −w "$PORT_NUMBER" # Is that particular port open? # grep −w matches whole words only, #+ so this wouldn't match port 1025, for example. exit 0 # 25/tcp
open
smtp
free Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more. bash$ free total Mem: 30504 −/+ buffers/cache: Swap: 68540
used 28624 10640 3128
free 1880 19864 65412
shared 15820
buffers 1608
cached 16376
To show unused RAM memory: bash$ free | grep Mem | awk '{ print $4 }' 1880
procinfo Extract and list information and statistics from the /proc pseudo−filesystem. This gives a very extensive and detailed listing. bash$ procinfo | grep Bootup Bootup: Wed Mar 21 15:15:50 2001
Load average: 0.04 0.21 0.34 3/47 6829
lsdev List devices, that is, show installed hardware. bash$ lsdev Device DMA IRQ I/O Ports −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− cascade 4 2
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Advanced Bash−Scripting Guide dma dma1 dma2 fpu ide0 ...
14
0080−008f 0000−001f 00c0−00df 00f0−00ff 01f0−01f7 03f6−03f6
du Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified. bash$ du −ach 1.0k ./wi.sh 1.0k ./tst.sh 1.0k ./random.file 6.0k . 6.0k total
df Shows filesystem usage in tabular form. bash$ df Filesystem /dev/hda5 /dev/hda8 /dev/hda7
1k−blocks 273262 222525 1408796
Used Available Use% Mounted on 92607 166547 36% / 123951 87085 59% /home 1075744 261488 80% /usr
stat Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files. bash$ stat test.cru File: "test.cru" Size: 49970 Allocated Blocks: 100 Filetype: Regular File Mode: (0664/−rw−rw−r−−) Uid: ( 501/ bozo) Gid: ( 501/ bozo) Device: 3,8 Inode: 18185 Links: 1 Access: Sat Jun 2 16:40:24 2001 Modify: Sat Jun 2 16:40:24 2001 Change: Sat Jun 2 16:40:24 2001
If the target file does not exist, stat returns an error message. bash$ stat nonexistent−file nonexistent−file: No such file or directory
vmstat Display virtual memory statistics. bash$ vmstat procs r b w swpd 0 0 0 0
free 11040
buff 2636
memory cache 38952
si 0
swap so 0
bi 33
io system bo in 7 271
cs 88
us 8
cpu sy id 3 89
netstat Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (Chapter 28). See Example 28−3. netstat −r is equivalent to route.
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Advanced Bash−Scripting Guide bash$ netstat Active Internet connections (w/o servers) Proto Recv−Q Send−Q Local Address Foreign Address State Active UNIX domain sockets (w/o servers) Proto RefCnt Flags Type State I−Node Path unix 11 [ ] DGRAM 906 /dev/log unix 3 [ ] STREAM CONNECTED 4514 /tmp/.X11−unix/X0 unix 3 [ ] STREAM CONNECTED 4513 . . .
uptime Shows how long the system has been running, along with associated statistics. bash$ uptime 10:28pm up 1:57,
3 users,
load average: 0.17, 0.34, 0.27
hostname Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname −n, and a counterpart to the $HOSTNAME internal variable. bash$ hostname localhost.localdomain bash$ echo $HOSTNAME localhost.localdomain
Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to hostname also perform these functions. hostid Echo a 32−bit hexadecimal numerical identifier for the host machine. bash$ hostid 7f0100
This command allegedly fetches a "unique" serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed. The network address of a typical non−networked Linux machine, is found in /etc/hosts. bash$ cat /etc/hosts 127.0.0.1
localhost.localdomain localhost
As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid. sar Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation (SCO) released sar as Open Source in June, 1999.
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Advanced Bash−Scripting Guide This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard. bash$ sar Linux 2.4.9 (brooks.seringas.fr) 10:30:00 10:40:00 10:50:00 11:00:00 Average:
CPU all all all all
%user 2.21 3.36 1.12 2.23
14:32:30
LINUX RESTART
15:00:00 15:10:00 15:20:00 15:30:00 Average:
CPU all all all all
%user 8.59 4.07 0.79 6.33
09/26/03 %nice 10.90 0.00 0.00 3.63
%system 65.48 72.36 80.77 72.87
%iowait 0.00 0.00 0.00 0.00
%idle 21.41 24.28 18.11 21.27
%nice 2.40 1.00 2.94 1.70
%system 17.47 11.95 7.56 14.71
%iowait 0.00 0.00 0.00 0.00
%idle 71.54 82.98 88.71 77.26
readelf Show information and statistics about a designated elf binary. This is part of the binutils package. bash$ readelf −h /bin/bash ELF Header: Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 Class: ELF32 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX − System V ABI Version: 0 Type: EXEC (Executable file) . . .
size The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers. bash$ size /bin/bash text data bss 495971 22496 17392
dec 535859
hex filename 82d33 /bin/bash
System Logs logger Appends a user−generated message to the system log (/var/log/messages). You do not have to be root to invoke logger. logger Experiencing instability in network connection at 23:10, 05/21. # Now, do a 'tail /var/log/messages'.
By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages. logger −t $0 −i Logging at line "$LINENO". # The "−t" option specifies the tag for the logger entry.
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Advanced Bash−Scripting Guide # The "−i" option records the process ID. # tail /var/log/message # ... # Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.
logrotate This utility manages the system log files, rotating, compressing, deleting, and/or mailing them, as appropriate. Usually cron runs logrotate on a daily basis. Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system−wide ones. Job Control ps Process Statistics: lists currently executing processes by owner and PID (process ID). This is usually invoked with ax options, and may be piped to grep or sed to search for a specific process (see Example 11−11 and Example 28−2). bash$ 295 ?
ps ax | grep sendmail S 0:00 sendmail: accepting connections on port 25
pstree Lists currently executing processes in "tree" format. The −p option shows the PIDs, as well as the process names. top Continuously updated display of most cpu−intensive processes. The −b option displays in text mode, so that the output may be parsed or accessed from a script. bash$ top −b 8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13 45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle Mem: 78396K av, 65468K used, 12928K free, 0K shrd, Swap: 157208K av, 0K used, 157208K free PID 848 1 2 ...
USER bozo root root
PRI 17 8 9
NI 0 0 0
SIZE 996 512 0
RSS SHARE STAT %CPU %MEM 996 800 R 5.6 1.2 512 444 S 0.0 0.6 0 0 SW 0.0 0.0
TIME 0:00 0:04 0:00
2352K buff 37244K cached
COMMAND top init keventd
nice Run a background job with an altered priority. Priorities run from 19 (lowest) to −20 (highest). Only root may set the negative (higher) priorities. Related commands are renice, snice, and skill. nohup Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process. pidof Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act on the PID of a process (not its name), it is sometimes necessary to identify that PID. The pidof command is the approximate counterpart to the $PPID internal variable.
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Advanced Bash−Scripting Guide bash$ pidof xclock 880
Example 13−5. pidof helps kill a process #!/bin/bash # kill−process.sh NOPROCESS=2 process=xxxyyyzzz # Use nonexistent process. # For demo purposes only... # ... don't want to actually kill any actual process with this script. # # If, for example, you wanted to use this script to logoff the Internet, # process=pppd t=`pidof $process` # Find pid (process id) of $process. # The pid is needed by 'kill' (can't 'kill' by program name). if [ −z "$t" ] # If process not present, 'pidof' returns null. then echo "Process $process was not running." echo "Nothing killed." exit $NOPROCESS fi kill $t
# May need 'kill −9' for stubborn process.
# Need a check here to see if process allowed itself to be killed. # Perhaps another " t=`pidof $process` ".
# This entire script could be replaced by # kill $(pidof −x process_name) # but it would not be as instructive. exit 0
fuser Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be invoked with the −k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services. cron Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab. Some flavors of Linux run crond, Matthew Dillon's version of cron. Process Control and Booting init The init command is the parent of all processes. Called in the final step of a bootup, init determines Chapter 13. System and Administrative Commands
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Advanced Bash−Scripting Guide the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only. telinit Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous − be certain you understand it well before using! runlevel Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single−user mode (1), in multi−user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file. halt, shutdown, reboot Command set to shut the system down, usually just prior to a power down. Network ifconfig Network interface configuration and tuning utility. bash$ ifconfig −a lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:10 errors:0 dropped:0 overruns:0 frame:0 TX packets:10 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:700 (700.0 b) TX bytes:700 (700.0 b)
The ifconfig command is most often used at bootup to set up the interfaces, or to shut them down when rebooting. # Code snippets from /etc/rc.d/init.d/network # ... # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 0 [ −x /sbin/ifconfig ] || exit 0 # ... for i in $interfaces ; do if ifconfig $i 2>/dev/null | grep −q "UP" >/dev/null 2>&1 ; then action "Shutting down interface $i: " ./ifdown $i boot fi # The GNU−specific "−q" option to "grep" means "quiet", i.e., producing no output. # Redirecting output to /dev/null is therefore not strictly necessary. # ... echo "Currently active devices:" echo `/sbin/ifconfig | grep ^[a−z] | awk '{print $1}'` # ^^^^^ should be quoted to prevent globbing. # The following also work. # echo $(/sbin/ifconfig | awk '/^[a−z]/ { print $1 })' # echo $(/sbin/ifconfig | sed −e 's/ .*//') # Thanks, S.C., for additional comments.
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Advanced Bash−Scripting Guide See also Example 30−6. iwconfig This is the command set for configuring a wireless network. It is the wireless equivalent of ifconfig, above. route Show info about or make changes to the kernel routing table. bash$ route Destination Gateway Genmask Flags pm3−67.bozosisp * 255.255.255.255 UH 127.0.0.0 * 255.0.0.0 U default pm3−67.bozosisp 0.0.0.0 UG
MSS Window 40 0 40 0 40 0
irtt Iface 0 ppp0 0 lo 0 ppp0
chkconfig Check network configuration. This command lists and manages the network services started at bootup in the /etc/rc?.d directory. Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of some Linux flavors. bash$ chkconfig −−list atd 0:off rwhod 0:off ...
1:off 1:off
2:off 2:off
3:on 3:off
4:on 4:off
5:on 5:off
6:off 6:off
tcpdump Network packet "sniffer". This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria. Dump ip packet traffic between hosts bozoville and caduceus: bash$ tcpdump ip host bozoville and caduceus
Of course, the output of tcpdump can be parsed, using certain of the previously discussed text processing utilities. Filesystem mount Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file /etc/fstab provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file /etc/mtab shows the currently mounted filesystems and partitions (including the virtual ones, such as /proc). mount −a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this to get everything mounted. mount −t iso9660 /dev/cdrom /mnt/cdrom # Mounts CDROM mount /mnt/cdrom # Shortcut, if /mnt/cdrom listed in /etc/fstab
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Advanced Bash−Scripting Guide This versatile command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 image before burning it onto a CDR. [40]
Example 13−6. Checking a CD image # As root... mkdir /mnt/cdtest
# Prepare a mount point, if not already there.
mount −r −t iso9660 −o loop cd−image.iso /mnt/cdtest # Mount the image. # "−o loop" option equivalent to "losetup /dev/loop0" cd /mnt/cdtest # Now, check the image. ls −alR # List the files in the directory tree there. # And so forth.
umount Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be umounted, else filesystem corruption may result. umount /mnt/cdrom # You may now press the eject button and safely remove the disk.
The automount utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On laptops with swappable floppy and CDROM drives, this can cause problems, though. sync Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot. At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see Example 12−48) or when the lights begin to flicker. losetup Sets up and configures loopback devices.
Example 13−7. Creating a filesystem in a file SIZE=1000000
# 1 meg
head −c $SIZE < /dev/zero > file losetup /dev/loop0 file mke2fs /dev/loop0 mount −o loop /dev/loop0 /mnt
# # # #
Set up file of designated size. Set it up as loopback device. Create filesystem. Mount it.
# Thanks, S.C.
mkswap Creates a swap partition or file. The swap area must subsequently be enabled with swapon. swapon, swapoff Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown. mke2fs Chapter 13. System and Administrative Commands
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Advanced Bash−Scripting Guide Create a Linux ext2 filesystem. This command must be invoked as root.
Example 13−8. Adding a new hard drive #!/bin/bash # # # #
Adding a second hard drive to system. Software configuration. Assumes hardware already mounted. From an article by the author of this document. In issue #38 of "Linux Gazette", http://www.linuxgazette.com.
ROOT_UID=0 E_NOTROOT=67
# This script must be run as root. # Non−root exit error.
if [ "$UID" −ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi # Use with extreme caution! # If something goes wrong, you may wipe out your current filesystem.
NEWDISK=/dev/hdb MOUNTPOINT=/mnt/newdisk
# Assumes /dev/hdb vacant. Check! # Or choose another mount point.
fdisk $NEWDISK mke2fs −cv $NEWDISK1 # Check for bad blocks & verbose output. # Note: /dev/hdb1, *not* /dev/hdb! mkdir $MOUNTPOINT chmod 777 $MOUNTPOINT # Makes new drive accessible to all users.
# # # #
Now, test... mount −t ext2 /dev/hdb1 /mnt/newdisk Try creating a directory. If it works, umount it, and proceed.
# Final step: # Add the following line to /etc/fstab. # /dev/hdb1 /mnt/newdisk ext2 defaults
1 1
exit 0
See also Example 13−7 and Example 29−3. tune2fs Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root. This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem. dumpe2fs Dump (list to stdout) very verbose filesystem info. This must be invoked as root. root# dumpe2fs /dev/hda7 | grep 'ount count'
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Advanced Bash−Scripting Guide dumpe2fs 1.19, 13−Jul−2000 for EXT2 FS 0.5b, 95/08/09 Mount count: 6 Maximum mount count: 20
hdparm List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused. fdisk Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root. Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem. fsck, e2fsck, debugfs Filesystem check, repair, and debug command set. fsck: a front end for checking a Unix filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2. e2fsck: ext2 filesystem checker. debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only! All of these should be invoked as root, and they can damage or destroy a filesystem if misused. badblocks Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. [41] As an example, badblocks /dev/fd0 tests a floppy disk. The badblocks command may be invoked destructively (overwrite all data) or in non−destructive read−only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command. lsusb, usbmodules The lsusb command lists all USB (Universal Serial Bus) buses and the devices hooked up to them. The usbmodules command outputs information about the driver modules for connected USB devices. root# lsusb Bus 001 Device 001: ID 0000:0000 Device Descriptor: bLength 18 bDescriptorType 1 bcdUSB 1.00 bDeviceClass 9 Hub bDeviceSubClass 0 bDeviceProtocol 0 bMaxPacketSize0 8 idVendor 0x0000 idProduct 0x0000 . . .
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Advanced Bash−Scripting Guide Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory. chroot CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a "chroot jail"). Note that after a chroot, the execution path for system binaries is no longer valid. A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as the base directory, rather than / as is normally the case. An alias XX 'chroot /aaa/bbb ls' in a user's ~/.bashrc effectively restricts which portion of the filesystem she may run command "XX" on. The chroot command is also handy when running from an emergency boot floppy (chroot to /dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care. It might be necessary to copy certain system files to a chrooted directory, since the normal $PATH can no longer be relied upon. lockfile This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphore file that controls access to a file, device, or resource. The lock file serves as a flag that this particular file, device, or resource is in use by a particular process ("busy"), and this permits only restricted access (or no access) to other processes. Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Netscape is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts create a lock file that already exists, the script will likely hang. Normally, applications create and check for lock files in the /var/lock directory. A script can test for the presence of a lock file by something like the following. appname=xyzip # Application "xyzip" created lock file "/var/lock/xyzip.lock". if [ −e "/var/lock/$appname.lock ] then ...
mknod Creates block or character device files (may be necessary when installing new hardware on the system). The MAKEDEV utility has virtually all of the functionality of mknod, and is easier to use. MAKEDEV Utility for creating device files. It must be run as root, and in the /dev directory. root# ./MAKEDEV
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Advanced Bash−Scripting Guide This is a sort of advanced version of mknod. tmpwatch Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by cron to remove stale log files. Backup dump, restore The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. [42] It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The restore command restores backups made with dump. fdformat Perform a low−level format on a floppy disk. System Resources ulimit Sets an upper limit on use of system resources. Usually invoked with the −f option, which sets a limit on file size (ulimit −f 1000 limits files to 1 meg maximum). The −t option limits the coredump size (ulimit −c 0 eliminates coredumps). Normally, the value of ulimit would be set in /etc/profile and/or ~/.bash_profile (see Chapter 27). Judicious use of ulimit can protect a system against the dreaded fork bomb. #!/bin/bash # This script is for illustrative purposes only. # Run it at your own peril −− it *will* freeze your system. while true do $0 &
#
Endless loop.
done
# #+ #+ #
This script invokes itself . . . forks an infinite number of times . . . until the system freezes up because all resources exhausted. This is the notorious "sorcerer's appentice" scenario.
exit 0
#
Will not exit here, because this script will never terminate.
A ulimit −Hu XX (where XX is the user process limit) in /etc/profile would abort this script when it exceeds the preset limit. setquota Set user or group disk quotas from the command line. umask User file creation MASK. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by umask. The (octal) value passed to umask defines the file permissions disabled. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022). [43] Of course, the user may later change the attributes of particular files with chmod. The usual practice is to set the value of umask in /etc/profile and/or ~/.bash_profile (see Chapter 27). rdev Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is a Chapter 13. System and Administrative Commands
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Advanced Bash−Scripting Guide dangerous command, if misused. Modules lsmod List installed kernel modules. bash$ lsmod Module autofs opl3 serial_cs sb uart401 sound soundlow soundcore ds i82365 pcmcia_core
Size Used by 9456 2 (autoclean) 11376 0 5456 0 (unused) 34752 0 6384 0 [sb] 58368 0 [opl3 sb uart401] 464 0 [sound] 2800 6 [sb sound] 6448 2 [serial_cs] 22928 2 45984 0 [serial_cs ds i82365]
Doing a cat /proc/modules gives the same information. insmod Force installation of a kernel module (use modprobe instead, when possible). Must be invoked as root. rmmod Force unloading of a kernel module. Must be invoked as root. modprobe Module loader that is normally invoked automatically in a startup script. Must be invoked as root. depmod Creates module dependency file, usually invoked from startup script. modinfo Output information about a loadable module. bash$ modinfo hid filename: /lib/modules/2.4.20−6/kernel/drivers/usb/hid.o description: "USB HID support drivers" author: "Andreas Gal, Vojtech Pavlik " license: "GPL"
Miscellaneous env Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The [varname=xxx] permits changing the environmental variable varname for the duration of the script. With no options specified, this command lists all the environmental variable settings. In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's environment.
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Advanced Bash−Scripting Guide var1=value1 var2=value2 commandXXX # $var1 and $var2 set in the environment of 'commandXXX' only.
The first line of a script (the "sha−bang" line) may use env when the path to the shell or interpreter is unknown. #! /usr/bin/env perl print "This Perl script will run,\n"; print "even when I don't know where to find Perl.\n"; # Good for portable cross−platform scripts, # where the Perl binaries may not be in the expected place. # Thanks, S.C.
ldd Show shared lib dependencies for an executable file. bash$ ldd /bin/ls libc.so.6 => /lib/libc.so.6 (0x4000c000) /lib/ld−linux.so.2 => /lib/ld−linux.so.2 (0x80000000)
watch Run a command repeatedly, at specified time intervals. The default is two−second intervals, but this may be changed with the −n option. watch −n 5 tail /var/log/messages # Shows tail end of system log, /var/log/messages, every five seconds.
strip Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging it impossible. This command often occurs in a Makefile, but rarely in a shell script. nm List symbols in an unstripped compiled binary. rdist Remote distribution client: synchronizes, clones, or backs up a file system on a remote server. Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is killall, used to suspend running processes at system shutdown.
Example 13−9. killall, from /etc/rc.d/init.d #!/bin/sh # −−> Comments added by the author of this document marked by "# −−>". # −−> This is part of the 'rc' script package # −−> by Miquel van Smoorenburg, # −−> This particular script seems to be Red Hat specific # −−> (may not be present in other distributions). # Bring down all unneeded services that are still running (there shouldn't
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Advanced Bash−Scripting Guide # be any, so this is just a sanity check) for i in /var/lock/subsys/*; do # −−> Standard for/in loop, but since "do" is on same line, # −−> it is necessary to add ";". # Check if the script is there. [ ! −f $i ] && continue # −−> This is a clever use of an "and list", equivalent to: # −−> if [ ! −f "$i" ]; then continue # Get the subsystem name. subsys=${i#/var/lock/subsys/} # −−> Match variable name, which, in this case, is the file name. # −−> This is the exact equivalent of subsys=`basename $i`. # −−> It gets it from the lock file name (if there is a lock file, # −−>+ that's proof the process has been running). # −−> See the "lockfile" entry, above.
# Bring the subsystem down. if [ −f /etc/rc.d/init.d/$subsys.init ]; then /etc/rc.d/init.d/$subsys.init stop else /etc/rc.d/init.d/$subsys stop # −−> Suspend running jobs and daemons # −−> using the 'stop' shell builtin. fi done
That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there. Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the −vn flags (sh −vn scriptname). Add extensive comments. Change the "action" commands to "echos". Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand parts of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts−?.??, which is part of the "initscripts" documentation.
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Chapter 14. Command Substitution Command substitution reassigns the output of a command [44] or even multiple commands; it literally plugs the command output into another context. [45] The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command line text. script_name=`basename $0` echo "The name of this script is $script_name."
The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for loop. rm `cat filename` # "filename" contains a list of files to delete. # # S. C. points out that "arg list too long" error might result. # Better is xargs rm −− < filename # ( −− covers those cases where "filename" begins with a "−" ) textfile_listing=`ls *.txt` # Variable contains names of all *.txt files in current working directory. echo $textfile_listing textfile_listing2=$(ls *.txt) echo $textfile_listing2 # Same result. # # # # # # # #
# The alternative form of command substitution.
A possible problem with putting a list of files into a single string is that a newline may creep in. A safer way to assign a list of files to a parameter is with an array. shopt −s nullglob # If no match, filename expands to nothing. textfile_listing=( *.txt ) Thanks, S.C.
Command substitution invokes a subshell. Command substitution may result in word splitting. COMMAND `echo a b`
# 2 args: a and b
COMMAND "`echo a b`"
# 1 arg: "a b"
COMMAND `echo`
# no arg
COMMAND "`echo`"
# one empty arg
# Thanks, S.C.
Even when there is no word splitting, command substitution can remove trailing newlines.
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Advanced Bash−Scripting Guide # cd "`pwd`" # However...
# This should always work.
mkdir 'dir with trailing newline ' cd 'dir with trailing newline ' cd "`pwd`" # Error message: # bash: cd: /tmp/file with trailing newline: No such file or directory cd "$PWD"
# Works fine.
old_tty_setting=$(stty −g) echo "Hit a key " stty −icanon −echo
# Save old terminal setting.
# Disable "canonical" mode for terminal. # Also, disable *local* echo. key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress. stty "$old_tty_setting" # Restore old setting. echo "You hit ${#key} key." # ${#variable} = number of characters in $variable # # Hit any key except RETURN, and the output is "You hit 1 key." # Hit RETURN, and it's "You hit 0 key." # The newline gets eaten in the command substitution. Thanks, S.C.
Using echo to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises. dir_listing=`ls −l` echo $dir_listing
# unquoted
# Expecting a nicely ordered directory listing. # However, what you get is: # total 3 −rw−rw−r−− 1 bozo bozo 30 May 13 17:15 1.txt −rw−rw−r−− 1 bozo # bozo 51 May 15 20:57 t2.sh −rwxr−xr−x 1 bozo bozo 217 Mar 5 21:13 wi.sh # The newlines disappeared.
echo "$dir_listing" # quoted # −rw−rw−r−− 1 bozo 30 May 13 17:15 1.txt # −rw−rw−r−− 1 bozo 51 May 15 20:57 t2.sh # −rwxr−xr−x 1 bozo 217 Mar 5 21:13 wi.sh
Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command. variable1=`/dev/null|grep −E "^I.*Cls=03.*Prot=02"` kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep −E "^I.*Cls=03.*Prot=01"` ... fi
Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke.
Example 14−1. Stupid script tricks #!/bin/bash # stupid−script−tricks.sh: Don't try this at home, folks. # From "Stupid Script Tricks," Volume I.
dangerous_variable=`cat /boot/vmlinuz`
# The compressed Linux kernel itself.
echo "string−length of \$dangerous_variable = ${#dangerous_variable}" # string−length of $dangerous_variable = 794151 # (Does not give same count as 'wc −c /boot/vmlinuz'.) # echo "$dangerous_variable" # Don't try this! It would hang the script.
# The document author is aware of no useful applications for #+ setting a variable to the contents of a binary file.
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Advanced Bash−Scripting Guide exit 0
Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as Bash, provides more protection from programmer mistakes than a compiled language. Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop.
Example 14−2. Generating a variable from a loop #!/bin/bash # csubloop.sh: Setting a variable to the output of a loop. variable1=`for i in 1 2 3 4 5 do echo −n "$i" done`
# The 'echo' command is critical #+ to command substitution here.
echo "variable1 = $variable1"
# variable1 = 12345
i=0 variable2=`while [ "$i" −lt 10 ] do echo −n "$i" # Again, the necessary 'echo'. let "i += 1" # Increment. done` echo "variable2 = $variable2"
# variable2 = 0123456789
# Demonstrates that it's possible to embed a loop #+ within a variable declaration. exit 0
Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of writing a program or script that outputs to stdout (like a well−behaved Unix tool should) and assigning that output to a variable. #include /*
"Hello, world." C program
*/
int main() { printf( "Hello, world." ); return (0); } bash$ gcc −o hello hello.c
#!/bin/bash # hello.sh
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Advanced Bash−Scripting Guide
greeting=`./hello` echo $greeting bash$ sh hello.sh Hello, world.
The $(COMMAND) form has superseded backticks for command substitution. output=$(sed −n /"$1"/p $file)
# From "grp.sh"
example.
# Setting a variable to the contents of a text file. File_contents1=$(cat $file1) File_contents2=$( # Redirect stdout to a file. # Creates the file if not present, otherwise overwrites it. ls −lR > dir−tree.list # Creates a file containing a listing of the directory tree. : > filename # The > truncates file "filename" to zero length. # If file not present, creates zero−length file (same effect as 'touch'). # The : serves as a dummy placeholder, producing no output. > filename # The > truncates file "filename" to zero length. # If file not present, creates zero−length file (same effect as 'touch'). # (Same result as ": >", above, but this does not work with some shells.) COMMAND_OUTPUT >> # Redirect stdout to a file. # Creates the file if not present, otherwise appends to it.
# Single−line redirection commands (affect only the line they are on): # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 1>filename # Redirect 1>>filename # Redirect 2>filename # Redirect 2>>filename # Redirect &>filename # Redirect
stdout to file "filename". and append stdout to file "filename". stderr to file "filename". and append stderr to file "filename". both stdout and stderr to file "filename".
#============================================================================== # Redirecting stdout, one line at a time. LOGFILE=script.log echo echo echo echo
"This "This "This "This
statement statement statement statement
Chapter 16. I/O Redirection
is is is is
sent to the log file, \"$LOGFILE\"." 1>$LOGFILE appended to \"$LOGFILE\"." 1>>$LOGFILE also appended to \"$LOGFILE\"." 1>>$LOGFILE echoed to stdout, and will not appear in \"$LOGFILE\"."
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Advanced Bash−Scripting Guide # These redirection commands automatically "reset" after each line.
# Redirecting stderr, one line at a time. ERRORFILE=script.errors bad_command1 2>$ERRORFILE bad_command2 2>>$ERRORFILE bad_command3
# Error message sent to $ERRORFILE. # Error message appended to $ERRORFILE. # Error message echoed to stderr, #+ and does not appear in $ERRORFILE. # These redirection commands also automatically "reset" after each line. #==============================================================================
2>&1 # Redirects stderr to stdout. # Error messages get sent to same place as standard output. i>&j # Redirects file descriptor i to j. # All output of file pointed to by i gets sent to file pointed to by j. >&j # Redirects, by default, file descriptor 1 (stdout) to j. # All stdout gets sent to file pointed to by j. 0< FILENAME < FILENAME # Accept input from a file. # Companion command to ">", and often used in combination with it. # # grep search−word File # Write string to "File". exec 3 File # Open "File" and assign fd 3 to it. read −n 4 &3 # Write a decimal point there. exec 3>&− # Close fd 3. cat File # ==> 1234.67890 # Random access, by golly.
| # Pipe. # General purpose process and command chaining tool. # Similar to ">", but more general in effect. # Useful for chaining commands, scripts, files, and programs together. cat *.txt | sort | uniq > result−file # Sorts the output of all the .txt files and deletes duplicate lines, # finally saves results to "result−file".
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Advanced Bash−Scripting Guide Multiple instances of input and output redirection and/or pipes can be combined in a single command line. command < input−file > output−file command1 | command2 | command3 > output−file
See Example 12−26 and Example A−16. Multiple output streams may be redirected to one file. ls # # #+
−yz >> command.log 2>&1 Capture result of illegal options "yz" in file "command.log." Because stderr is redirected to the file, any error messages will also be there.
# Note, however, that the following does *not* give the same result. ls −yz 2>&1 >> command.log # Outputs an error message and does not write to file. # If redirecting both stdout and stderr, #+ the order of the commands makes a difference.
Closing File Descriptors n&− Close stdout. Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it. # Redirecting only stderr to a pipe. exec 3>&1 ls −l 2>&1 >&3 3>&− | grep bad 3>&− # ^^^^ ^^^^ exec 3>&−
# Save current "value" of stdout. # Close fd 3 for 'grep' (but not 'ls'). # Now close it for the remainder of the script.
# Thanks, S.C.
For a more detailed introduction to I/O redirection see Appendix E.
16.1. Using exec An exec &7 7>&− exec 0&2;;
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Advanced Bash−Scripting Guide *) exec 3> /dev/null 4> /dev/null 5> /dev/null;; esac FD_LOGVARS=6 if [[ $LOG_VARS ]] then exec 6>> /var/log/vars.log else exec 6> /dev/null fi
# Bury output.
FD_LOGEVENTS=7 if [[ $LOG_EVENTS ]] then # then exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log) # Above line will not work in Bash, version 2.04. exec 7>> /var/log/event.log # Append to "event.log". log # Write time and date. else exec 7> /dev/null # Bury output. fi echo "DEBUG3: beginning" >&${FD_DEBUG3} ls −l >&5 2>&4 echo "Done"
# command1 >&5 2>&4
echo "sending mail" >&${FD_LOGEVENTS}
# command2 # Writes "sending mail" to fd #7.
exit 0
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Chapter 17. Here Documents A here document is a special−purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or command, such as ftp, telnet, or ex. COMMAND file.tar.bz2& tar cf pipe $directory_name rm pipe # or exec 3>&1 tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&− | bzip2 −c > file.tar.bz2 3>&− exec 3>&−
# Thanks, Stepane Chazelas
A reader of this document sent in the following interesting example of process substitution. # Script fragment taken from SuSE distribution: while read des what mask iface; do # Some commands ... done < 0)) in # Must have at least one disk. 1) dohanoi $1 1 3 2 echo "Total moves = $Moves" exit 0; ;; *) echo "$0: illegal value for number of disks"; exit $E_BADPARAM; ;; esac ;; *) echo "usage: $0 N" echo " Where \"N\" is the number of disks." exit $E_NOPARAM; ;; esac # # # # # #
Exercises: −−−−−−−−− 1) Would commands beyond this point ever be executed? Why not? (Easy) 2) Explain the workings of the workings of the "dohanoi" function. (Difficult)
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Chapter 24. Aliases A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence. If, for example, we include alias lm="ls −l | more" in the ~/.bashrc file, then each lm typed at the command line will automatically be replaced by a ls −l | more. This can save a great deal of typing at the command line and avoid having to remember complex combinations of commands and options. Setting alias rm="rm −i" (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently losing important files. In a script, aliases have very limited usefulness. It would be quite nice if aliases could assume some of the functionality of the C preprocessor, such as macro expansion, but unfortunately Bash does not expand arguments within the alias body. [55] Moreover, a script fails to expand an alias itself within "compound constructs", such as if/then statements, loops, and functions. An added limitation is that an alias will not expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function.
Example 24−1. Aliases within a script #!/bin/bash # Invoke with command line parameter to exercise last section of this script. shopt −s expand_aliases # Must set this option, else script will not expand aliases.
# First, some fun. alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."' Jesse_James echo; echo; echo; alias ll="ls −l" # May use either single (') or double (") quotes to define an alias. echo "Trying aliased \"ll\":" ll /usr/X11R6/bin/mk* #* Alias works. echo directory=/usr/X11R6/bin/ prefix=mk* # See if wild−card causes problems. echo "Variables \"directory\" + \"prefix\" = $directory$prefix" echo alias lll="ls −l $directory$prefix" echo "Trying aliased \"lll\":" lll # Long listing of all files in /usr/X11R6/bin stating with mk. # Alias handles concatenated variables, including wild−card o.k.
TRUE=1
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Advanced Bash−Scripting Guide echo if [ TRUE ] then alias rr="ls −l" echo "Trying aliased \"rr\" within if/then statement:" rr /usr/X11R6/bin/mk* #* Error message results! # Aliases not expanded within compound statements. echo "However, previously expanded alias still recognized:" ll /usr/X11R6/bin/mk* fi echo count=0 while [ $count −lt 3 ] do alias rrr="ls −l" echo "Trying aliased \"rrr\" within \"while\" loop:" rrr /usr/X11R6/bin/mk* #* Alias will not expand here either. # alias.sh: line 57: rrr: command not found let count+=1 done echo; echo alias xyz='cat $0'
# Script lists itself. # Note strong quotes.
xyz # This seems to work, #+ although the Bash documentation suggests that it shouldn't. # # However, as Steve Jacobson points out, #+ the "$0" parameter expands immediately upon declaration of the alias. exit 0
The unalias command removes a previously set alias.
Example 24−2. unalias: Setting and unsetting an alias #!/bin/bash shopt −s expand_aliases
# Enables alias expansion.
alias llm='ls −al | more' llm echo unalias llm # Unset alias. llm # Error message results, since 'llm' no longer recognized. exit 0 bash$ ./unalias.sh total 6 drwxrwxr−x 2 bozo
Chapter 24. Aliases
bozo
3072 Feb
6 14:04 .
312
Advanced Bash−Scripting Guide drwxr−xr−x −rwxr−xr−x
40 bozo 1 bozo
bozo bozo
2048 Feb 199 Feb
6 14:04 .. 6 14:04 unalias.sh
./unalias.sh: llm: command not found
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Chapter 25. List Constructs The "and list" and "or list" constructs provide a means of processing a number of commands consecutively. These can effectively replace complex nested if/then or even case statements. Chaining together commands and list command−1 && command−2 && command−3 && ... command−n
Each command executes in turn provided that the previous command has given a return value of true (zero). At the first false (non−zero) return, the command chain terminates (the first command returning false is the last one to execute). Example 25−1. Using an "and list" to test for command−line arguments #!/bin/bash # "and list" if [ ! −z "$1" ] && echo "Argument #1 = $1" && [ ! −z "$2" ] && echo "Argument #2 = $2" then echo "At least 2 arguments passed to script." # All the chained commands return true. else echo "Less than 2 arguments passed to script." # At least one of the chained commands returns false. fi # Note that "if [ ! −z $1 ]" works, but its supposed equivalent, # if [ −n $1 ] does not. However, quoting fixes this. # if [ −n "$1" ] works. Careful! # It is best to always quote tested variables.
# This if [ ! then echo fi if [ ! then echo echo else echo fi # It's
accomplishes the same thing, using "pure" if/then statements. −z "$1" ] "Argument #1 = $1" −z "$2" ] "Argument #2 = $2" "At least 2 arguments passed to script." "Less than 2 arguments passed to script." longer and less elegant than using an "and list".
exit 0
Example 25−2. Another command−line arg test using an "and list"
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Advanced Bash−Scripting Guide #!/bin/bash ARGS=1 E_BADARGS=65
# Number of arguments expected. # Exit value if incorrect number of args passed.
test $# −ne $ARGS && echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS # If condition−1 true (wrong number of args passed to script), # then the rest of the line executes, and script terminates. # Line below executes only if the above test fails. echo "Correct number of arguments passed to this script." exit 0 # To check exit value, do a "echo $?" after script termination.
Of course, an and list can also set variables to a default value. arg1=$@
# Set $arg1 to command line arguments, if any.
[ −z "$arg1" ] && arg1=DEFAULT # Set to DEFAULT if not specified on command line.
or list command−1 || command−2 || command−3 || ... command−n
Each command executes in turn for as long as the previous command returns false. At the first true return, the command chain terminates (the first command returning true is the last one to execute). This is obviously the inverse of the "and list". Example 25−3. Using "or lists" in combination with an "and list" #!/bin/bash # #
delete.sh, not−so−cunning file deletion utility. Usage: delete filename
E_BADARGS=65 if [ −z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS # No arg? Bail out. else file=$1 # Set filename. fi
[ ! −f "$file" ] && echo "File \"$file\" not found. \ Cowardly refusing to delete a nonexistent file." # AND LIST, to give error message if file not present. # Note echo message continued on to a second line with an escape. [ ! −f "$file" ] || (rm −f $file; echo "File \"$file\" deleted.") # OR LIST, to delete file if present. # Note logic inversion above.
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Advanced Bash−Scripting Guide # AND LIST executes on true, OR LIST on false. exit 0
If the first command in an "or list" returns true, it will execute. The exit status of an and list or an or list is the exit status of the last command executed. Clever combinations of "and" and "or" lists are possible, but the logic may easily become convoluted and require extensive debugging. false && true || echo false # Same result as ( false && true ) || echo false # But *not* false && ( true || echo false )
# false
# false # (nothing echoed)
# Note left−to−right grouping and evaluation of statements, #+ since the logic operators "&&" and "||" have equal precedence. #
It's best to avoid such complexities, unless you know what you're doing.
#
Thanks, S.C.
See Example A−8 and Example 7−4 for illustrations of using an and / or list to test variables.
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Chapter 26. Arrays Newer versions of Bash support one−dimensional arrays. Array elements may be initialized with the variable[xx] notation. Alternatively, a script may introduce the entire array by an explicit declare −a variable statement. To dereference (find the contents of) an array element, use curly bracket notation, that is, ${variable[xx]}.
Example 26−1. Simple array usage #!/bin/bash
area[11]=23 area[13]=37 area[51]=UFOs # Array members need not be consecutive or contiguous. # Some members of the array can be left uninitialized. # Gaps in the array are o.k.
echo −n "area[11] = " echo ${area[11]} #
{curly brackets} needed
echo −n "area[13] = " echo ${area[13]} echo "Contents of area[51] are ${area[51]}." # Contents of uninitialized array variable print blank. echo −n "area[43] = " echo ${area[43]} echo "(area[43] unassigned)" echo # Sum of two array variables assigned to third area[5]=`expr ${area[11]} + ${area[13]}` echo "area[5] = area[11] + area[13]" echo −n "area[5] = " echo ${area[5]} area[6]=`expr ${area[11]} + ${area[51]}` echo "area[6] = area[11] + area[51]" echo −n "area[6] = " echo ${area[6]} # This fails because adding an integer to a string is not permitted. echo; echo; echo # # # #
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Another array, "area2". Another way of assigning array variables... array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
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Advanced Bash−Scripting Guide echo −n "area2[0] = " echo ${area2[0]} # Aha, zero−based indexing (first element of array is [0], not [1]). echo −n "area2[1] = " echo ${area2[1]} # [1] is second element of array. # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− echo; echo; echo # # # #
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Yet another array, "area3". Yet another way of assigning array variables... array_name=([xx]=XXX [yy]=YYY ...)
area3=([17]=seventeen [24]=twenty−four) echo −n "area3[17] = " echo ${area3[17]} echo −n "area3[24] = " echo ${area3[24]} # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− exit 0
Bash permits array operations on variables, even if the variables are not explicitly declared as arrays. string=abcABC123ABCabc echo ${string[@]} echo ${string[*]} echo ${string[0]} echo ${string[1]} echo ${#string[@]}
# # # # # # # #
abcABC123ABCabc abcABC123ABCabc abcABC123ABCabc No output! Why? 1 One element in the array. The string itself.
# Thank you, Michael Zick, for pointing this out.
Once again this demonstrates that Bash variables are untyped.
Example 26−2. Formatting a poem #!/bin/bash # poem.sh: Pretty−prints one of the document author's favorite poems. # Lines of the poem (single stanza). Line[1]="I do not know which to prefer," Line[2]="The beauty of inflections" Line[3]="Or the beauty of innuendoes," Line[4]="The blackbird whistling" Line[5]="Or just after." # Attribution. Attrib[1]=" Wallace Stevens"
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Advanced Bash−Scripting Guide Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\"" # This poem is in the Public Domain (copyright expired). echo for index in 1 2 3 4 5 # Five lines. do printf " %s\n" "${Line[index]}" done for index in 1 2 do printf " done
# Two attribution lines. %s\n" "${Attrib[index]}"
echo exit 0 # Exercise: # −−−−−−−− # Modify this script to pretty−print a poem from a text data file.
Array variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use.
Example 26−3. Various array operations #!/bin/bash # array−ops.sh: More fun with arrays.
array=( zero one two three four five ) # Element 0 1 2 3 4 5 echo ${array[0]} echo ${array:0}
# # # #+ # # #+
zero zero Parameter expansion of starting at position # ero Parameter expansion of starting at position #
# # # # #
4 Length of first element of array. 4 Length of first element of array. (Alternate notation)
echo ${#array[1]}
# # #
3 Length of second element of array. Arrays in Bash have zero−based indexing.
echo ${#array[*]}
# # # #
6 Number of elements in array. 6 Number of elements in array.
echo ${array:1}
first element, 0 (1st character). first element, 1 (2nd character).
echo "−−−−−−−−−−−−−−" echo ${#array[0]} echo ${#array}
echo ${#array[@]}
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Advanced Bash−Scripting Guide echo "−−−−−−−−−−−−−−" array2=( [0]="first element" [1]="second element" [3]="fourth element" ) echo ${array2[0]} echo ${array2[1]} echo ${array2[2]} echo ${array2[3]}
# # # # #
first element second element Skipped in initialization, and therefore null. fourth element
exit 0
Many of the standard string operations work on arrays.
Example 26−4. String operations on arrays #!/bin/bash # array−strops.sh: String operations on arrays. # Script by Michael Zick. # Used with permission. # In general, any string operation in the ${name ... } notation #+ can be applied to all string elements in an array #+ with the ${name[@] ... } or ${name[*] ...} notation.
arrayZ=( one two three four five five ) echo # Trailing Substring Extraction echo ${arrayZ[@]:0} # one two three four five five # All elements. echo ${arrayZ[@]:1}
# two three four five five # All elements following element[0].
echo ${arrayZ[@]:1:2}
# two three # Only the two elements after element[0].
echo "−−−−−−−−−−−−−−−−−−−−−−−" # Substring Removal # Removes shortest match from front of string(s), #+ where the substring is a regular expression. echo ${arrayZ[@]#f*r}
# one two three five five # Applied to all elements of the array. # Matches "four" and removes it.
# Longest match from front of string(s) echo ${arrayZ[@]##t*e} # one two four five five # Applied to all elements of the array. # Matches "three" and removes it. # Shortest match from back of string(s) echo ${arrayZ[@]%h*e} # one two t four five five
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Advanced Bash−Scripting Guide # Applied to all elements of the array. # Matches "hree" and removes it. # Longest match from back echo ${arrayZ[@]%%t*e} # # #
of string(s) one two four five five Applied to all elements of the array. Matches "three" and removes it.
echo "−−−−−−−−−−−−−−−−−−−−−−−" # Substring Replacement # Replace first occurance of substring with replacement echo ${arrayZ[@]/fiv/XYZ} # one two three four XYZe XYZe # Applied to all elements of the array. # Replace all occurances of substring echo ${arrayZ[@]//iv/YY} # one two three four fYYe fYYe # Applied to all elements of the array. # Delete all occurances of substring # Not specifing a replacement means 'delete' echo ${arrayZ[@]//fi/} # one two three four ve ve # Applied to all elements of the array. # Replace front−end occurances of substring echo ${arrayZ[@]/#fi/XY} # one two three four XYve XYve # Applied to all elements of the array. # Replace back−end occurances of substring echo ${arrayZ[@]/%ve/ZZ} # one two three four fiZZ fiZZ # Applied to all elements of the array. echo ${arrayZ[@]/%o/XX}
# one twXX three four five five # Why?
echo "−−−−−−−−−−−−−−−−−−−−−−−"
# Before reaching for awk (or anything else) −− # Recall: # $( ... ) is command substitution. # Functions run as a sub−process. # Functions write their output to stdout. # Assignment reads the function's stdout. # The name[@] notation specifies a "for−each" operation. newstr() { echo −n "!!!" } echo ${arrayZ[@]/%e/$(newstr)} # on!!! two thre!!! four fiv!!! fiv!!! # Q.E.D: The replacement action is an 'assignment.' # Accessing the "For−Each" echo ${arrayZ[@]//*/$(newstr optional_arguments)} # Now, if Bash would just pass the matched string as $0 #+ to the function being called . . . echo
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Advanced Bash−Scripting Guide exit 0
Command substitution can construct the individual elements of an array.
Example 26−5. Loading the contents of a script into an array #!/bin/bash # script−array.sh: Loads this script into an array. # Inspired by an e−mail from Chris Martin (thanks!). script_contents=( $(cat "$0") )
# Stores contents of this script ($0) #+ in an array.
for element in $(seq 0 $((${#script_contents[@]} − 1))) do # ${#script_contents[@]} #+ gives number of elements in the array. # # Question: # Why is seq 0 necessary? # Try changing it to seq 1. echo −n "${script_contents[$element]}" # List each field of this script on a single line. echo −n " −− " # Use " −− " as a field separator. done echo exit 0 # Exercise: # −−−−−−−− # Modify this script so it lists itself #+ in its original format, #+ complete with whitespace, line breaks, etc.
In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array elements, or even an entire array.
Example 26−6. Some special properties of arrays #!/bin/bash declare −a colors # All subsequent commands in this script will treat #+ the variable "colors" as an array. echo "Enter your favorite colors (separated from each other by a space)." read −a colors # Enter at least 3 colors to demonstrate features below. # Special option to 'read' command, #+ allowing assignment of elements in an array. echo element_count=${#colors[@]} # Special syntax to extract number of elements in array. # element_count=${#colors[*]} works also.
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Advanced Bash−Scripting Guide # # #+ # # #+
The "@" variable allows word splitting within quotes (extracts variables separated by whitespace). This corresponds to the behavior of "$@" and "$*" in positional parameters.
index=0 while [ "$index" −lt "$element_count" ] do # List all the elements in the array. echo ${colors[$index]} let "index = $index + 1" done # Each array element listed on a separate line. # If this is not desired, use echo −n "${colors[$index]} " # # Doing it with a "for" loop instead: # for i in "${colors[@]}" # do # echo "$i" # done # (Thanks, S.C.) echo # Again, list all the elements in the array, but using a more elegant method. echo ${colors[@]} # echo ${colors[*]} also works. echo # The "unset" command deletes elements of an array, or entire array. unset colors[1] # Remove 2nd element of array. # Same effect as colors[1]= echo ${colors[@]} # List array again, missing 2nd element. unset colors
# Delete entire array. # unset colors[*] and #+ unset colors[@] also work.
echo; echo −n "Colors gone." echo ${colors[@]} # List array again, now empty. exit 0
As seen in the previous example, either ${array_name[@]} or ${array_name[*]} refers to all the elements of the array. Similarly, to get a count of the number of elements in an array, use either ${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length (number of characters) of ${array_name[0]}, the first element of the array.
Example 26−7. Of empty arrays and empty elements #!/bin/bash # empty−array.sh # Thanks to Stephane Chazelas for the original example, #+ and to Michael Zick for extending it.
# An empty array is not the same as an array with empty elements.
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Advanced Bash−Scripting Guide array0=( first second third ) array1=( '' ) # "array1" consists of one empty element. array2=( ) # No elements . . . "array2" is empty. echo ListArray() { echo echo "Elements in array0: ${array0[@]}" echo "Elements in array1: ${array1[@]}" echo "Elements in array2: ${array2[@]}" echo echo "Length of first element in array0 = ${#array0}" echo "Length of first element in array1 = ${#array1}" echo "Length of first element in array2 = ${#array2}" echo echo "Number of elements in array0 = ${#array0[*]}" # 3 echo "Number of elements in array1 = ${#array1[*]}" # 1 echo "Number of elements in array2 = ${#array2[*]}" # 0 }
(Surprise!)
# =================================================================== ListArray # Try extending those arrays. # Adding array0=( array1=( array2=(
an element to an array. "${array0[@]}" "new1" ) "${array1[@]}" "new1" ) "${array2[@]}" "new1" )
ListArray # or array0[${#array0[*]}]="new2" array1[${#array1[*]}]="new2" array2[${#array2[*]}]="new2" ListArray # When extended as above; arrays are 'stacks' # The above is the 'push' # The stack 'height' is: height=${#array2[@]} echo echo "Stack height for array2 = $height" # The 'pop' is: unset array2[${#array2[@]}−1] # Arrays are zero−based, height=${#array2[@]} #+ which means first element has index 0. echo echo "POP" echo "New stack height for array2 = $height" ListArray # List only 2nd and 3rd elements of array0. from=1 # Zero−based numbering. to=2 # array3=( ${array0[@]:1:2} )
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Advanced Bash−Scripting Guide echo echo "Elements in array3:
${array3[@]}"
# Works like a string (array of characters). # Try some other "string" forms. # Replacement: array4=( ${array0[@]/second/2nd} ) echo echo "Elements in array4: ${array4[@]}" # Replace all matching wildcarded string. array5=( ${array0[@]//new?/old} ) echo echo "Elements in array5: ${array5[@]}" # Just when you are getting the feel for this . . . array6=( ${array0[@]#*new} ) echo # This one might surprise you. echo "Elements in array6: ${array6[@]}" array7=( ${array0[@]#new1} ) echo # After array6 this should not be a surprise. echo "Elements in array7: ${array7[@]}" # Which looks a lot like . . . array8=( ${array0[@]/new1/} ) echo echo "Elements in array8: ${array8[@]}" #
So what can one say about this?
# #+ # #+ #+
The string operations are performed on each of the elements in var[@] in succession. Therefore : Bash supports string vector operations if the result is a zero length string, that element disappears in the resulting assignment.
#
Question, are those strings hard or soft quotes?
zap='new*' array9=( ${array0[@]/$zap/} ) echo echo "Elements in array9: ${array9[@]}" # Just when you thought you where still in Kansas . . . array10=( ${array0[@]#$zap} ) echo echo "Elements in array10: ${array10[@]}" # Compare array7 with array10. # Compare array8 with array9. # Answer: must be soft quotes. exit 0
The relationship of ${array_name[@]} and ${array_name[*]} is analogous to that between $@ and $*. This powerful array notation has a number of uses.
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Advanced Bash−Scripting Guide # Copying an array. array2=( "${array1[@]}" ) # or array2="${array1[@]}" # Adding an element to an array. array=( "${array[@]}" "new element" ) # or array[${#array[*]}]="new element" # Thanks, S.C.
The array=( element1 element2 ... elementN ) initialization operation, with the help of command substitution, makes it possible to load the contents of a text file into an array. #!/bin/bash filename=sample_file # # # #
cat sample_file 1 a b c 2 d e fg
declare −a array1 array1=( `cat "$filename"`) # Loads contents # List file to stdout #+ of $filename into array1. # # array1=( `cat "$filename" | tr '\n' ' '`) # change linefeeds in file to spaces. # Not necessary because Bash does word splitting, #+ changing linefeeds to spaces. echo ${array1[@]} # List the array. # 1 a b c 2 d e fg # # Each whitespace−separated "word" in the file #+ has been assigned to an element of the array. element_count=${#array1[*]} echo $element_count
# 8
Clever scripting makes it possible to add array operations.
Example 26−8. Initializing arrays #! /bin/bash # array−assign.bash # Array operations are Bash specific, #+ hence the ".bash" in the script name. # Copyright (c) Michael S. Zick, 2003, All rights reserved. # License: Unrestricted reuse in any form, for any purpose. # Version: $ID$ #
Chapter 26. Arrays
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Advanced Bash−Scripting Guide # Clarification and additional comments by William Park. # Based on an example provided by Stephane Chazelas #+ which appeared in the book: Advanced Bash Scripting Guide. # Output format of the 'times' command: # User CPU System CPU # User CPU of dead children System CPU of dead children # #+ # #+ # #+
Bash has two versions of assigning all elements of an array to a new array variable. Both drop 'null reference' elements in Bash versions 2.04, 2.05a and 2.05b. An additional array assignment that maintains the relationship of [subscript]=value for arrays may be added to newer versions.
# Constructs a large array using an internal command, #+ but anything creating an array of several thousand elements #+ will do just fine. declare −a bigOne=( /dev/* ) echo echo 'Conditions: Unquoted, default IFS, All−Elements−Of' echo "Number of elements in array is ${#bigOne[@]}" # set −vx
echo echo '− − testing: =( ${array[@]} ) − −' times declare −a bigTwo=( ${bigOne[@]} ) # ^ ^ times echo echo '− − testing: =${array[@]} − −' times declare −a bigThree=${bigOne[@]} # No parentheses this time. times # #+ # # #+ #+ # # #
Comparing the numbers shows that the second form, pointed out by Stephane Chazelas, is from three to four times faster. William Park explains: Second method is assigning bigOne[] as single string, whereas first method is assigning bigOne[] element by element. So, in essence, you So, in essence, you have: bigTwo=( [0]="... ... ..." ) bigThree=( [0]="..." [1]="..." [2]="..." ... )
# I will continue to use the first form in my example descriptions #+ because I think it is a better illustration of what is happening. # The reusable portions of my examples will actual contain #+ the second form where appropriate because of the speedup. # MSZ: Sorry about that earlier oversight folks.
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Advanced Bash−Scripting Guide # # # #+ #+ # #+ #
Note: −−−− The "declare −a" statements in lines 31 are not strictly necessary, since it is in the Array=( ... ) assignment form. However, eliminating these declarations the execution of the following sections Try it, and see what happens.
and 43 implicit slows down of the script.
exit 0
Adding a superfluous declare −a statement to an array declaration may speed up execution of subsequent operations on the array.
Example 26−9. Copying and concatenating arrays #! /bin/bash # CopyArray.sh # # This script written by Michael Zick. # Used here with permission. # How−To "Pass by Name & Return by Name" #+ or "Building your own assignment statement".
CpArray_Mac() { # Assignment Command Statement Builder echo echo echo echo echo
−n −n −n −n −n
'eval ' "$2" '=( ${' "$1" '[@]} )'
# Destination name # Source name
# That could all be a single command. # Matter of style only. } declare −f CopyArray CopyArray=CpArray_Mac
# Function "Pointer" # Statement Builder
Hype() { # Hype the array named $1. # (Splice it together with array containing "Really Rocks".) # Return in array named $2. local −a TMP local −a hype=( Really Rocks ) $($CopyArray $1 TMP) TMP=( ${TMP[@]} ${hype[@]} ) $($CopyArray TMP $2) }
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Advanced Bash−Scripting Guide declare −a before=( Advanced Bash Scripting ) declare −a after echo "Array Before = ${before[@]}" Hype before after echo "Array After = ${after[@]}" # Too much hype? echo "What ${after[@]:3:2}?" declare −a modest=( ${after[@]:2:1} ${after[@]:3:2} ) # −−−− substring extraction −−−− echo "Array Modest = ${modest[@]}" # What happened to 'before' ? echo "Array Before = ${before[@]}" exit 0
Example 26−10. More on concatenating arrays #! /bin/bash # array−append.bash # # # # #
Copyright (c) Michael S. Zick, 2003, All rights reserved. License: Unrestricted reuse in any form, for any purpose. Version: $ID$ Slightly modified in formatting by M.C.
# Array operations are Bash−specific. # Legacy UNIX /bin/sh lacks equivalents.
# Pipe the output of this script to 'more' #+ so it doesn't scroll off the terminal.
# Subscript packed. declare −a array1=( zero1 one1 two1 ) # Subscript sparse ([1] is not defined). declare −a array2=( [0]=zero2 [2]=two2 [3]=three2 ) echo echo '− Confirm that the array is really subscript sparse. −' echo "Number of elements: 4" # Hard−coded for illustration. for (( i = 0 ; i < 4 ; i++ )) do echo "Element [$i]: ${array2[$i]}" done # See also the more general code example in basics−reviewed.bash.
declare −a dest
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Advanced Bash−Scripting Guide # Combine (append) two arrays into a third array. echo echo 'Conditions: Unquoted, default IFS, All−Elements−Of operator' echo '− Undefined elements not present, subscripts not maintained. −' # # The undefined elements do not exist; they are not being dropped. dest=( ${array1[@]} ${array2[@]} ) # dest=${array1[@]}${array2[@]}
# Strange results, possibly a bug.
# Now, list the result. echo echo '− − Testing Array Append − −' cnt=${#dest[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${dest[$i]}" done # Assign an array to a single array element (twice). dest[0]=${array1[@]} dest[1]=${array2[@]} # List the result. echo echo '− − Testing modified array − −' cnt=${#dest[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${dest[$i]}" done # Examine the modified second element. echo echo '− − Reassign and list second element − −' declare −a subArray=${dest[1]} cnt=${#subArray[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${subArray[$i]}" done # #+ #+ #+
The assignment of an entire array to a of another array using the '=${ ... }' has converted the array being assigned with the elements separated by a space
single element array assignment into a string, (the first character of IFS).
# If the original elements didn't contain whitespace . . . # If the original array isn't subscript sparse . . . # Then we could get the original array structure back again. # Restore from the modified second element. echo echo '− − Listing restored element − −'
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Advanced Bash−Scripting Guide declare −a subArray=( ${dest[1]} ) cnt=${#subArray[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${subArray[$i]}" done echo '− − Do not depend on this behavior. − −' echo '− − This behavior is subject to change − −' echo '− − in versions of Bash newer than version 2.05b − −' # MSZ: Sorry about any earlier confusion folks. exit 0
−− Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left to the reader to decide.
Example 26−11. An old friend: The Bubble Sort #!/bin/bash # bubble.sh: Bubble sort, of sorts. # Recall the algorithm for a bubble sort. In this particular version... # #+ # # # # #
With each successive pass through the array to be sorted, compare two adjacent elements, and swap them if out of order. At the end of the first pass, the "heaviest" element has sunk to bottom. At the end of the second pass, the next "heaviest" one has sunk next to bottom. And so forth. This means that each successive pass needs to traverse less of the array. You will therefore notice a speeding up in the printing of the later passes.
exchange() { # Swaps two members of the array. local temp=${Countries[$1]} # Temporary storage #+ for element getting swapped out. Countries[$1]=${Countries[$2]} Countries[$2]=$temp return } declare −a Countries
# Declare array, #+ optional here since it's initialized below.
# Is it permissable to split an array variable over multiple lines #+ using an escape (\)? # Yes. Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \ Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \ Israel Peru Canada Oman Denmark Wales France Kenya \ Xanadu Qatar Liechtenstein Hungary)
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Advanced Bash−Scripting Guide # "Xanadu" is the mythical place where, according to Coleridge, #+ Kubla Khan did a pleasure dome decree.
clear
# Clear the screen to start with.
echo "0: ${Countries[*]}"
# List entire array at pass 0.
number_of_elements=${#Countries[@]} let "comparisons = $number_of_elements − 1" count=1 # Pass number. while [ "$comparisons" −gt 0 ] do index=0
# Beginning of outer loop
# Reset index to start of array after each pass.
while [ "$index" −lt "$comparisons" ] # Beginning of inner loop do if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ] # If out of order... # Recalling that \> is ASCII comparison operator #+ within single brackets. # if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]] #+ also works. then exchange $index `expr $index + 1` # Swap. fi let "index += 1" done # End of inner loop # # # # # # # # # # # #
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Paulo Marcel Coelho Aragao suggests for−loops as a simpler altenative. for (( last = $number_of_elements − 1 ; last > 1 ; last−− )) do for (( i = 0 ; i < last ; i++ )) do [[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \ && exchange $i $((i+1)) done done −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
let "comparisons −= 1" # Since "heaviest" element bubbles to bottom, #+ we need do one less comparison each pass. echo echo "$count: ${Countries[@]}" echo let "count += 1" done
# Print resultant array at end of each pass. # Increment pass count. # End of outer loop # All done.
exit 0
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Advanced Bash−Scripting Guide −− Is it possible to nest arrays within arrays? #!/bin/bash # "Nested" array. # Michael Zick provided this example, #+ with corrections and clarifications by William Park. AnArray=( $(ls −−inode −−ignore−backups −−almost−all \ −−directory −−full−time −−color=none −−time=status \ −−sort=time −l ${PWD} ) ) # Commands and options. # Spaces are significant . . . and don't quote anything in the above. SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} ) # This array has six elements: #+ SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]} # [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} ) # # Arrays in Bash are (circularly) linked lists #+ of type string (char *). # So, this isn't actually a nested array, #+ but it's functionally similar. echo "Current directory and date of last status change:" echo "${SubArray[@]}" exit 0
−− Embedded arrays in combination with indirect references create some fascinating possibilities
Example 26−12. Embedded arrays and indirect references #!/bin/bash # embedded−arrays.sh # Embedded arrays and indirect references. # This script by Dennis Leeuw. # Used with permission. # Modified by document author.
ARRAY1=( VAR1_1=value11 VAR1_2=value12 VAR1_3=value13 ) ARRAY2=( VARIABLE="test" STRING="VAR1=value1 VAR2=value2 VAR3=value3" ARRAY21=${ARRAY1[*]} ) # Embed ARRAY1 within this second array.
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Advanced Bash−Scripting Guide function print () { OLD_IFS="$IFS" IFS=$'\n'
# To print each array element #+ on a separate line. TEST1="ARRAY2[*]" local ${!TEST1} # See what happens if you delete this line. # Indirect reference. # This makes the components of $TEST1 #+ accessible to this function.
# Let's see what we've got so far. echo echo "\$TEST1 = $TEST1" # Just the name of the variable. echo; echo echo "{\$TEST1} = ${!TEST1}" # Contents of the variable. # That's what an indirect #+ reference does. echo echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"; echo echo
# Print variable echo "Variable VARIABLE: $VARIABLE" # Print a string element IFS="$OLD_IFS" TEST2="STRING[*]" local ${!TEST2} # Indirect reference (as above). echo "String element VAR2: $VAR2 from STRING" # Print an array element TEST2="ARRAY21[*]" local ${!TEST2} # Indirect reference (as above). echo "Array element VAR1_1: $VAR1_1 from ARRAY21" } print echo exit 0 # As the author of the script notes, #+ "you can easily expand it to create named−hashes in bash." # (Difficult) exercise for the reader: implement this.
−− Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a resource−intensive application of this nature should really be written in a compiled language, such as C. It runs excruciatingly slowly as a script.
Example 26−13. Complex array application: Sieve of Eratosthenes #!/bin/bash # sieve.sh # Sieve of Eratosthenes
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Advanced Bash−Scripting Guide # Ancient algorithm for finding prime numbers. # This runs a couple of orders of magnitude # slower than the equivalent C program. LOWER_LIMIT=1 # Starting with 1. UPPER_LIMIT=1000 # Up to 1000. # (You may set this higher... if you have time on your hands.) PRIME=1 NON_PRIME=0 let SPLIT=UPPER_LIMIT/2 # Optimization: # Need to test numbers only halfway to upper limit.
declare −a Primes # Primes[] is an array.
initialize () { # Initialize the array. i=$LOWER_LIMIT until [ "$i" −gt "$UPPER_LIMIT" ] do Primes[i]=$PRIME let "i += 1" done # Assume all array members guilty (prime) # until proven innocent. } print_primes () { # Print out the members of the Primes[] array tagged as prime. i=$LOWER_LIMIT until [ "$i" −gt "$UPPER_LIMIT" ] do if [ "${Primes[i]}" −eq "$PRIME" ] then printf "%8d" $i # 8 spaces per number gives nice, even columns. fi let "i += 1" done } sift () # Sift out the non−primes. { let i=$LOWER_LIMIT+1 # We know 1 is prime, so let's start with 2.
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Advanced Bash−Scripting Guide until [ "$i" −gt "$UPPER_LIMIT" ] do if [ "${Primes[i]}" −eq "$PRIME" ] # Don't bother sieving numbers already sieved (tagged as non−prime). then t=$i while [ "$t" −le "$UPPER_LIMIT" ] do let "t += $i " Primes[t]=$NON_PRIME # Tag as non−prime all multiples. done fi let "i += 1" done
}
# Invoke the functions sequentially. initialize sift print_primes # This is what they call structured programming. echo exit 0
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # # Code below line will not execute. # This improved version of the Sieve, by Stephane Chazelas, # executes somewhat faster. # Must invoke with command−line argument (limit of primes). UPPER_LIMIT=$1 let SPLIT=UPPER_LIMIT/2
# From command line. # Halfway to max number.
Primes=( '' $(seq $UPPER_LIMIT) ) i=1 until (( ( i += 1 ) > SPLIT )) # Need check only halfway. do if [[ −n $Primes[i] ]] then t=$i until (( ( t += i ) > UPPER_LIMIT )) do Primes[t]= done fi done
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Advanced Bash−Scripting Guide echo ${Primes[*]} exit 0
Compare this array−based prime number generator with an alternative that does not use arrays, Example A−17. −− Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support.
Example 26−14. Emulating a push−down stack #!/bin/bash # stack.sh: push−down stack simulation # Similar to the CPU stack, a push−down stack stores data items #+ sequentially, but releases them in reverse order, last−in first−out. BP=100
# Base Pointer of stack array. # Begin at element 100.
SP=$BP
# Stack Pointer. # Initialize it to "base" (bottom) of stack.
Data=
# Contents of stack location. # Must use local variable, #+ because of limitation on function return range.
declare −a stack
push() { if [ −z "$1" ] then return fi
# Push item on stack.
let "SP −= 1" stack[$SP]=$1
# Bump stack pointer.
# Nothing to push?
return } pop() { Data=
# Pop item off stack.
if [ "$SP" −eq "$BP" ] then return fi
# Stack empty?
Data=${stack[$SP]} let "SP += 1" return
Chapter 26. Arrays
# Empty out data item.
# This also keeps SP from getting past 100, #+ i.e., prevents a runaway stack.
# Bump stack pointer.
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Advanced Bash−Scripting Guide } status_report() # Find out what's happening. { echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" echo "REPORT" echo "Stack Pointer = $SP" echo "Just popped \""$Data"\" off the stack." echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" echo }
# ======================================================= # Now, for some fun. echo # See if you can pop anything off empty stack. pop status_report echo push garbage pop status_report
# Garbage in, garbage out.
value1=23; push $value1 value2=skidoo; push $value2 value3=FINAL; push $value3 pop status_report pop status_report pop status_report
# FINAL # skidoo # 23 # Last−in, first−out!
# Notice how the stack pointer decrements with each push, #+ and increments with each pop. echo # =======================================================
# Exercises: # −−−−−−−−− # 1) Modify the "push()" function to permit pushing # + multiple element on the stack with a single function call. # 2) Modify the "pop()" function to permit popping # + multiple element from the stack with a single function call. # 3) Using this script as a jumping−off point, # + write a stack−based 4−function calculator. exit 0
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Advanced Bash−Scripting Guide Fancy manipulation of array "subscripts" may require intermediate variables. For projects involving this, again consider using a more powerful programming language, such as Perl or C.
Example 26−15. Complex array application: Exploring a weird mathematical series #!/bin/bash # Douglas Hofstadter's notorious "Q−series": # Q(1) = Q(2) = 1 # Q(n) = Q(n − Q(n−1)) + Q(n − Q(n−2)), for n>2 # This is a "chaotic" integer series with strange and unpredictable behavior. # The first 20 terms of the series are: # 1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12 # See Hofstadter's book, "Goedel, Escher, Bach: An Eternal Golden Braid", # p. 137, ff.
LIMIT=100 LINEWIDTH=20
# Number of terms to calculate # Number of terms printed per line
Q[1]=1 Q[2]=1
# First two terms of series are 1.
echo echo "Q−series [$LIMIT terms]:" echo −n "${Q[1]} " # Output first two terms. echo −n "${Q[2]} " for ((n=3; n 2 # Need to break the expression into intermediate terms, # since Bash doesn't handle complex array arithmetic very well. let "n1 = $n − 1" let "n2 = $n − 2"
# n−1 # n−2
t0=`expr $n − ${Q[n1]}` t1=`expr $n − ${Q[n2]}`
# n − Q[n−1] # n − Q[n−2]
T0=${Q[t0]} T1=${Q[t1]}
# Q[n − Q[n−1]] # Q[n − Q[n−2]]
Q[n]=`expr $T0 + $T1` echo −n "${Q[n]} "
# Q[n − Q[n−1]] + Q[n − Q[n−2]]
if [ `expr $n % $LINEWIDTH` −eq 0 ] # Format output. then # mod echo # Break lines into neat chunks. fi done echo exit 0 # This is an iterative implementation of the Q−series.
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Advanced Bash−Scripting Guide # The more intuitive recursive implementation is left as an exercise. # Warning: calculating this series recursively takes a *very* long time.
−− Bash supports only one−dimensional arrays, however a little trickery permits simulating multi−dimensional ones.
Example 26−16. Simulating a two−dimensional array, then tilting it #!/bin/bash # twodim.sh: Simulating a two−dimensional array. # A one−dimensional array consists of a single row. # A two−dimensional array stores rows sequentially. Rows=5 Columns=5 # 5 X 5 Array. declare −a alpha
# char alpha [Rows] [Columns]; # Unnecessary declaration. Why?
load_alpha () { local rc=0 local index for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y do # Use different symbols if you like. local row=`expr $rc / $Columns` local column=`expr $rc % $Rows` let "index = $row * $Rows + $column" alpha[$index]=$i # alpha[$row][$column] let "rc += 1" done # Simpler would be #+ declare −a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y ) #+ but this somehow lacks the "flavor" of a two−dimensional array. } print_alpha () { local row=0 local index echo while [ "$row" −lt "$Rows" ] do
# Print out in "row major" order: #+ columns vary, #+ while row (outer loop) remains the same.
local column=0 echo −n "
"
#
Lines up "square" array with rotated one.
while [ "$column" −lt "$Columns" ] do
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Advanced Bash−Scripting Guide let "index = $row * $Rows + $column" echo −n "${alpha[index]} " # alpha[$row][$column] let "column += 1" done let "row += 1" echo done # The simpler equivalent is # echo ${alpha[*]} | xargs −n $Columns echo } filter () { echo −n "
# Filter out negative array indices.
"
# Provides the tilt. # Explain why.
if [[ "$1" −ge 0 && "$1" −lt "$Rows" && "$2" −ge 0 && "$2" −lt "$Columns" ]] then let "index = $1 * $Rows + $2" # Now, print it rotated. echo −n " ${alpha[index]}" # alpha[$row][$column] fi }
rotate () # Rotate the array 45 degrees −− { #+ "balance" it on its lower lefthand corner. local row local column for (( row = Rows; row > −Rows; row−− )) do # Step through the array backwards. Why? for (( column = 0; column < Columns; column++ )) do if [ "$row" then let "t1 = let "t2 = else let "t1 = let "t2 = fi
−ge 0 ] $column − $row" $column" $column" $column + $row"
filter $t1 $t2 done
# Filter out negative array indices. Why?
echo; echo done
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Advanced Bash−Scripting Guide # #+ #+ # #+
Array rotation inspired by examples (pp. 143−146) in "Advanced C Programming on the IBM PC," by Herbert Mayer (see bibliography). This just goes to show that much of what can be done in C can also be done in shell scripting.
}
#−−−−−−−−−−−−−−− Now, let the show begin. −−−−−−−−−−−−# load_alpha # Load the array. print_alpha # Print it out. rotate # Rotate it 45 degrees counterclockwise. #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−# exit 0 # This is a rather contrived, not to mention inelegant simulation. # # # # # # # # # # #
Exercises: −−−−−−−−− 1) Rewrite the array loading and printing functions in a more intuitive and less kludgy fashion. 2)
Figure out how the array rotation functions work. Hint: think about the implications of backwards−indexing an array.
3)
Rewrite this script to handle a non−square array, such as a 6 X 4 one. Try to minimize "distortion" when the array is rotated.
A two−dimensional array is essentially equivalent to a one−dimensional one, but with additional addressing modes for referencing and manipulating the individual elements by "row" and "column" position. For an even more elaborate example of simulating a two−dimensional array, see Example A−11.
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Chapter 27. Files startup files These files contain the aliases and environmental variables made available to Bash running as a user shell and to all Bash scripts invoked after system initialization. /etc/profile systemwide defaults, mostly setting the environment (all Bourne−type shells, not just Bash [56]) /etc/bashrc systemwide functions and aliases for Bash $HOME/.bash_profile user−specific Bash environmental default settings, found in each user's home directory (the local counterpart to /etc/profile) $HOME/.bashrc user−specific Bash init file, found in each user's home directory (the local counterpart to /etc/bashrc). Only interactive shells and user scripts read this file. See Appendix J for a sample .bashrc file. logout file $HOME/.bash_logout user−specific instruction file, found in each user's home directory. Upon exit from a login (Bash) shell, the commands in this file execute.
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Chapter 28. /dev and /proc A Linux or Unix machine typically has the /dev and /proc special−purpose directories.
28.1. /dev The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [57] The hard drive partitions containing the mounted filesystem(s) have entries in /dev, as a simple df shows. bash$ df Filesystem Mounted on /dev/hda6 /dev/hda1 /dev/hda8 /dev/hda5
1k−blocks 495876 50755 367013 1714416
Used Available Use% 222748 3887 13262 1123624
247527 44248 334803 503704
48% 9% 4% 70%
/ /boot /home /usr
Among other things, the /dev directory also contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. [58] This enables mounting an entire filesystem within a single large file. See Example 13−7 and Example 13−6. A few of the pseudo−devices in /dev have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom, /dev/sda1, /dev/udp, and /dev/tcp. For instance: To mount a USB flash drive, append the following line to /etc/fstab. [59] /dev/sda1
/mnt/flashdrive auto
noauto,user
0 0
(See also Example A−22.)
When executing a command on a /dev/tcp/$host/$port pseudo−device file, Bash opens a TCP connection to the associated socket. [60] Getting the time from from nist.gov: bash$ cat &5" bash$ cat /dev/tcp/${TCP_HOST}/${TCP_PORT} MYEXIT=$? : From the bash reference: /dev/tcp/host/port If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. EXPLANATION
if [ "X$MYEXIT" = "X0" ]; then echo "Connection successful. Exit code: $MYEXIT" else echo "Connection unsuccessful. Exit code: $MYEXIT" fi exit $MYEXIT
28.2. /proc The /proc directory is actually a pseudo−filesystem. The files in /proc mirror currently running system and kernel processes and contain information and statistics about them. bash$ cat /proc/devices Character devices: 1 mem 2 pty 3 ttyp 4 ttyS 5 cua 7 vcs 10 misc 14 sound 29 fb
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Advanced Bash−Scripting Guide 36 128 136 162 254
netlink ptm pts raw pcmcia
Block devices: 1 ramdisk 2 fd 3 ide0 9 md
bash$ cat /proc/interrupts CPU0 0: 84505 XT−PIC 1: 3375 XT−PIC 2: 0 XT−PIC 5: 1 XT−PIC 8: 1 XT−PIC 12: 4231 XT−PIC 14: 109373 XT−PIC NMI: 0 ERR: 0
bash$ cat /proc/partitions major minor #blocks name 3 3 3 3 ...
0 1 2 4
3007872 52416 1 165280
timer keyboard cascade soundblaster rtc PS/2 Mouse ide0
rio rmerge rsect ruse wio wmerge wsect wuse running use aveq
hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030 hda1 27 395 844 960 4 2 14 180 0 800 1140 hda2 0 0 0 0 0 0 0 0 0 0 0 hda4 10 0 20 210 0 0 0 0 0 210 210
bash$ cat /proc/loadavg 0.13 0.42 0.27 2/44 1119
bash$ cat /proc/apm 1.16 1.2 0x03 0x01 0xff 0x80 −1% −1 ?
Shell scripts may extract data from certain of the files in /proc. [61] FS=iso
# ISO filesystem support in kernel?
grep $FS /proc/filesystems
# iso9660
kernel_version=$( awk '{ print $3 }' /proc/version ) CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo ) if [ $CPU = Pentium ] then
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Advanced Bash−Scripting Guide run_some_commands ... else run_different_commands ... fi
The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The stat and status files keep running statistics on the process, the cmdline file holds the command−line arguments the process was invoked with, and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint.
Example 28−2. Finding the process associated with a PID #!/bin/bash # pid−identifier.sh: Gives complete path name to process associated with pid. ARGNO=1 # Number of arguments the script expects. E_WRONGARGS=65 E_BADPID=66 E_NOSUCHPROCESS=67 E_NOPERMISSION=68 PROCFILE=exe if [ $# −ne $ARGNO ] then echo "Usage: `basename $0` PID−number" >&2 exit $E_WRONGARGS fi
# Error message >stderr.
pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 ) # Checks for pid in "ps" listing, field #1. # Then makes sure it is the actual process, not the process invoked by this script. # The last "grep $1" filters out this possibility. if [ −z "$pidno" ] # If, after all the filtering, the result is a zero−length string, then # no running process corresponds to the pid given. echo "No such process running." exit $E_NOSUCHPROCESS fi # Alternatively: # if ! ps $1 > /dev/null 2>&1 # then # no running process corresponds to the pid given. # echo "No such process running." # exit $E_NOSUCHPROCESS # fi # To simplify the entire process, use "pidof".
if [ ! then echo echo exit fi
−r "/proc/$1/$PROCFILE" ]
# Check for read permission.
"Process $1 running, but..." "Can't get read permission on /proc/$1/$PROCFILE." $E_NOPERMISSION # Ordinary user can't access some files in /proc.
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Advanced Bash−Scripting Guide # The last two tests may be replaced by: # if ! kill −0 $1 > /dev/null 2>&1 # '0' is not a signal, but # this will test whether it is possible # to send a signal to the process. # then echo "PID doesn't exist or you're not its owner" >&2 # exit $E_BADPID # fi
exe_file=$( ls −l /proc/$1 | grep "exe" | awk '{ print $11 }' ) # Or exe_file=$( ls −l /proc/$1/exe | awk '{print $11}' ) # # /proc/pid−number/exe is a symbolic link # to the complete path name of the invoking process. if [ −e "$exe_file" ] # If /proc/pid−number/exe exists... then # the corresponding process exists. echo "Process #$1 invoked by $exe_file." else echo "No such process running." fi
# # # # # # # # #
This elaborate script can *almost* be replaced by ps ax | grep $1 | awk '{ print $5 }' However, this will not work... because the fifth field of 'ps' is argv[0] of the process, not the executable file path. However, either of the following would work. find /proc/$1/exe −printf '%l\n' lsof −aFn −p $1 −d txt | sed −ne 's/^n//p'
# Additional commentary by Stephane Chazelas. exit 0
Example 28−3. On−line connect status #!/bin/bash PROCNAME=pppd PROCFILENAME=status NOTCONNECTED=65 INTERVAL=2
# ppp daemon # Where to look. # Update every 2 seconds.
pidno=$( ps ax | grep −v "ps ax" | grep −v grep | grep $PROCNAME | awk '{ print $1 }' ) # Finding the process number of 'pppd', the 'ppp daemon'. # Have to filter out the process lines generated by the search itself. # # However, as Oleg Philon points out, #+ this could have been considerably simplified by using "pidof". # pidno=$( pidof $PROCNAME ) # # Moral of the story: #+ When a command sequence gets too complex, look for a shortcut.
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Advanced Bash−Scripting Guide if [ −z "$pidno" ] # If no pid, then process is not running. then echo "Not connected." exit $NOTCONNECTED else echo "Connected."; echo fi while [ true ] do
# Endless loop, script can be improved here.
if [ ! −e "/proc/$pidno/$PROCFILENAME" ] # While process running, then "status" file exists. then echo "Disconnected." exit $NOTCONNECTED fi netstat −s | grep "packets received" # Get some connect statistics. netstat −s | grep "packets delivered"
sleep $INTERVAL echo; echo done exit 0 # As it stands, this script must be terminated with a Control−C. # # # #
Exercises: −−−−−−−−− Improve the script so it exits on a "q" keystroke. Make the script more user−friendly in other ways.
In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the machine.
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Chapter 29. Of Zeros and Nulls /dev/zero and /dev/null Uses of /dev/null Think of /dev/null as a "black hole". It is the nearest equivalent to a write−only file. Everything written to it disappears forever. Attempts to read or output from it result in nothing. Nevertheless, /dev/null can be quite useful from both the command line and in scripts. Suppressing stdout. cat $filename >/dev/null # Contents of the file will not list to stdout.
Suppressing stderr (from Example 12−3). rm $badname 2>/dev/null # So error messages [stderr] deep−sixed.
Suppressing output from both stdout and stderr. cat $filename 2>/dev/null >/dev/null # If "$filename" does not exist, there will be no error message output. # If "$filename" does exist, the contents of the file will not list to stdout. # Therefore, no output at all will result from the above line of code. # # This can be useful in situations where the return code from a command #+ needs to be tested, but no output is desired. # # cat $filename &>/dev/null # also works, as Baris Cicek points out.
Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example 2−1 and Example 2−3): cat /dev/null > /var/log/messages # : > /var/log/messages has same effect, but does not spawn a new process. cat /dev/null > /var/log/wtmp
Automatically emptying the contents of a logfile (especially good for dealing with those nasty "cookies" sent by Web commercial sites):
Example 29−1. Hiding the cookie jar if [ −f ~/.netscape/cookies ] then rm −f ~/.netscape/cookies fi
# Remove, if exists.
ln −s /dev/null ~/.netscape/cookies
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Advanced Bash−Scripting Guide # All cookies now get sent to a black hole, rather than saved to disk.
Uses of /dev/zero Like /dev/null, /dev/zero is a pseudo file, but it actually contains nulls (binary zeros, not the ASCII kind). Output written to it disappears, and it is fairly difficult to actually read the nulls in /dev/zero, though it can be done with od or a hex editor. The chief use for /dev/zero is in creating an initialized dummy file of specified length intended as a temporary swap file.
Example 29−2. Setting up a swapfile using /dev/zero #!/bin/bash # Creating a swapfile. # This script must be run as root. ROOT_UID=0 E_WRONG_USER=65
# Root has $UID 0. # Not root?
FILE=/swap BLOCKSIZE=1024 MINBLOCKS=40 SUCCESS=0 if [ "$UID" −ne "$ROOT_UID" ] then echo; echo "You must be root to run this script."; echo exit $E_WRONG_USER fi
blocks=${1:−$MINBLOCKS} # # # # # # # # #
# Set to default of 40 blocks, #+ if nothing specified on command line. This is the equivalent of the command block below. −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− if [ −n "$1" ] then blocks=$1 else blocks=$MINBLOCKS fi −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
if [ "$blocks" −lt $MINBLOCKS ] then blocks=$MINBLOCKS fi
# Must be at least 40 blocks long.
echo "Creating swap file of size $blocks blocks (KB)." dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file. mkswap $FILE $blocks swapon $FILE
# Designate it a swap file. # Activate swap file.
echo "Swap file created and activated." exit $SUCCESS
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Advanced Bash−Scripting Guide Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 13−7) or securely deleting a file (see Example 12−48).
Example 29−3. Creating a ramdisk #!/bin/bash # ramdisk.sh # #+ # # #+ # # # #+
A "ramdisk" is a segment of system RAM memory which acts as if it were a filesystem. Its advantage is very fast access (read/write time). Disadvantages: volatility, loss of data on reboot or powerdown. less RAM available to system. Of what use is a ramdisk? Keeping a large dataset, such as a table or dictionary on ramdisk, speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 ROOTUSER_NAME=root MOUNTPT=/mnt/ramdisk SIZE=2000 BLOCKSIZE=1024 DEVICE=/dev/ram0
# Must run as root.
# 2K blocks (change as appropriate) # 1K (1024 byte) block size # First ram device
username=`id −nu` if [ "$username" != "$ROOTUSER_NAME" ] then echo "Must be root to run \"`basename $0`\"." exit $E_NON_ROOT_USER fi if [ ! −d "$MOUNTPT" ] then mkdir $MOUNTPT fi
# Test whether mount point already there, #+ so no error if this script is run #+ multiple times.
dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE mke2fs $DEVICE mount $DEVICE $MOUNTPT chmod 777 $MOUNTPT
# # # #
# Zero out RAM device. # Why is this necessary? Create an ext2 filesystem on it. Mount it. Enables ordinary user to access ramdisk. However, must be root to unmount it.
echo "\"$MOUNTPT\" now available for use." # The ramdisk is now accessible for storing files, even by an ordinary user. # Caution, the ramdisk is volatile, and its contents will disappear #+ on reboot or power loss. # Copy anything you want saved to a regular directory. # After reboot, run this script to again set up ramdisk. # Remounting /mnt/ramdisk without the other steps will not work. # Suitably modified, this script can by invoked in /etc/rc.d/rc.local, #+ to set up ramdisk automatically at bootup.
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Advanced Bash−Scripting Guide #
That may be appropriate on, for example, a database server.
exit 0
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Chapter 30. Debugging The Bash shell contains no debugger, nor even any debugging−specific commands or constructs. [62] Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non−functional script.
Example 30−1. A buggy script #!/bin/bash # ex74.sh # This is a buggy script. # Where, oh where is the error? a=37 if [$a −gt 27 ] then echo $a fi exit 0
Output from script: ./ex74.sh: [37: command not found
What's wrong with the above script (hint: after the if)? Example 30−2. Missing keyword #!/bin/bash # missing−keyword.sh: What error message will this generate? for a in 1 2 3 do echo "$a" # done # Required keyword 'done' commented out in line 7. exit 0
Output from script: missing−keyword.sh: line 10: syntax error: unexpected end of file
Note that the error message does not necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error. Error messages may disregard comment lines in a script when reporting the line number of a syntax error. What if the script executes, but does not work as expected? This is the all too familiar logic error.
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Advanced Bash−Scripting Guide Example 30−3. test24, another buggy script #!/bin/bash # #+ # #
This script is supposed to delete all filenames in current directory containing embedded spaces. It doesn't work. Why not?
badname=`ls | grep ' '` # Try this: # echo "$badname" rm "$badname" exit 0
Try to find out what's wrong with Example 30−3 by uncommenting the echo "$badname" line. Echo statements are useful for seeing whether what you expect is actually what you get. In this particular case, rm "$badname" will not give the desired results because $badname should not be quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n'. However, there are simpler ways of going about it. # Correct methods of deleting filenames containing spaces. rm *\ * rm *" "* rm *' '* # Thank you. S.C.
Summarizing the symptoms of a buggy script, 1. It bombs with a "syntax error" message, or 2. It runs, but does not work as expected (logic error). 3. It runs, works as expected, but has nasty side effects (logic bomb). Tools for debugging non−working scripts include 1. echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on. 2. using the tee filter to check processes or data flows at critical points. 3. setting option flags −n −v −x sh −n scriptname checks for syntax errors without actually running the script. This is the equivalent of inserting set −n or set −o noexec into the script. Note that certain types of syntax errors can slip past this check. sh −v scriptname echoes each command before executing it. This is the equivalent of inserting set −v or set −o verbose in the script. The −n and −v flags work well together. sh −nv scriptname gives a verbose syntax check. Chapter 30. Debugging
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Advanced Bash−Scripting Guide sh −x scriptname echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting set −x or set −o xtrace in the script. Inserting set −u or set −o nounset in the script runs it, but gives an unbound variable error message at each attempt to use an undeclared variable. 4. Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.) Example 30−4. Testing a condition with an "assert" #!/bin/bash # assert.sh assert () { E_PARAM_ERR=98 E_ASSERT_FAILED=99
if [ −z "$2" ] then return $E_PARAM_ERR fi
# If condition false, #+ exit from script with error message.
# Not enough parameters passed. # No damage done.
lineno=$2 if [ ! $1 ] then echo "Assertion failed: \"$1\"" echo "File \"$0\", line $lineno" exit $E_ASSERT_FAILED # else # return # and continue executing script. fi }
a=5 b=4 condition="$a −lt $b"
# Error message and exit from script. # Try setting "condition" to something else, #+ and see what happens.
assert "$condition" $LINENO # The remainder of the script executes only if the "assert" does not fail.
# Some commands. # ... echo "This statement echoes only if the \"assert\" does not fail." # ... # Some more commands. exit 0
5. trapping at exit. The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [63] It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the Chapter 30. Debugging
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Advanced Bash−Scripting Guide first command in the script. Trapping signals trap Specifies an action on receipt of a signal; also useful for debugging. A signal is simply a message sent to a process, either by the kernel or another process, telling it to take some specified action (usually to terminate). For example, hitting a Control−C, sends a user interrupt, an INT signal, to a running program. trap '' 2 # Ignore interrupt 2 (Control−C), with no action specified. trap 'echo "Control−C disabled."' 2 # Message when Control−C pressed.
Example 30−5. Trapping at exit #!/bin/bash # Hunting variables with a trap. trap 'echo Variable Listing −−− a = $a b = $b' EXIT # EXIT is the name of the signal generated upon exit from a script. # # The command specified by the "trap" doesn't execute until #+ the appropriate signal is sent. echo "This prints before the \"trap\" −−" echo "even though the script sees the \"trap\" first." echo a=39 b=36 exit 0 # Note that commenting out the 'exit' command makes no difference, #+ since the script exits in any case after running out of commands.
Example 30−6. Cleaning up after Control−C #!/bin/bash # logon.sh: A quick 'n dirty script to check whether you are on−line yet.
TRUE=1 LOGFILE=/var/log/messages # Note that $LOGFILE must be readable #+ (as root, chmod 644 /var/log/messages). TEMPFILE=temp.$$ # Create a "unique" temp file name, using process id of the script. KEYWORD=address # At logon, the line "remote IP address xxx.xxx.xxx.xxx" # appended to /var/log/messages.
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Advanced Bash−Scripting Guide ONLINE=22 USER_INTERRUPT=13 CHECK_LINES=100 # How many lines in log file to check. trap 'rm −f $TEMPFILE; exit $USER_INTERRUPT' TERM INT # Cleans up the temp file if script interrupted by control−c. echo while [ $TRUE ] #Endless loop. do tail −$CHECK_LINES $LOGFILE> $TEMPFILE # Saves last 100 lines of system log file as temp file. # Necessary, since newer kernels generate many log messages at log on. search=`grep $KEYWORD $TEMPFILE` # Checks for presence of the "IP address" phrase, #+ indicating a successful logon. if [ ! −z "$search" ] # then echo "On−line" rm −f $TEMPFILE # exit $ONLINE else echo −n "." # #+ fi
Quotes necessary because of possible spaces.
Clean up temp file.
The −n option to echo suppresses newline, so you get continuous rows of dots.
sleep 1 done
# Note: if you change the KEYWORD variable to "Exit", #+ this script can be used while on−line #+ to check for an unexpected logoff. # Exercise: Change the script, per the above note, # and prettify it. exit 0
# Nick Drage suggests an alternate method: while true do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0 echo −n "." # Prints dots (.....) until connected. sleep 2 done # Problem: Hitting Control−C to terminate this process may be insufficient. #+ (Dots may keep on echoing.) # Exercise: Fix this.
# Stephane Chazelas has yet another alternative: CHECK_INTERVAL=1 while ! tail −1 "$LOGFILE" | grep −q "$KEYWORD"
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Advanced Bash−Scripting Guide do echo −n . sleep $CHECK_INTERVAL done echo "On−line" # Exercise: Discuss the relative strengths and weaknesses #! of each of these various approaches.
The DEBUG argument to trap causes a specified action to execute after every command in a script. This permits tracing variables, for example. Example 30−7. Tracing a variable #!/bin/bash trap 'echo "VARIABLE−TRACE> \$variable = \"$variable\""' DEBUG # Echoes the value of $variable after every command. variable=29 echo "Just initialized \"\$variable\" to $variable." let "variable *= 3" echo "Just multiplied \"\$variable\" by 3." # # # #
The "trap 'commands' DEBUG" construct would be more useful in the context of a complex script, where placing multiple "echo $variable" statements might be clumsy and time−consuming.
# Thanks, Stephane Chazelas for the pointer. exit 0
Of course, the trap command has other uses aside from debugging.
Example 30−8. Running multiple processes (on an SMP box) #!/bin/bash # multiple−processes.sh: Run multiple processes on an SMP box. # Script written by Vernia Damiano. # Used with permission. # Must call script with at least one integer parameter #+ (number of concurrent processes). # All other parameters are passed through to the processes started.
INDICE=8 TEMPO=5 E_BADARGS=65
# Total number of process to start # Maximum sleep time per process # No arg(s) passed to script.
if [ $# −eq 0 ] # Check for at least one argument passed to script. then echo "Usage: `basename $0` number_of_processes [passed params]" exit $E_BADARGS fi
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Advanced Bash−Scripting Guide NUMPROC=$1 shift PARAMETRI=( "$@" )
# Number of concurrent process # Parameters of each process
function avvia() { local temp local index temp=$RANDOM index=$1 shift let "temp %= $TEMPO" let "temp += 1" echo "Starting $index Time:$temp" "$@" sleep ${temp} echo "Ending $index" kill −s SIGRTMIN $$ } function parti() { if [ $INDICE −gt 0 ] ; then avvia $INDICE "${PARAMETRI[@]}" & let "INDICE−−" else trap : SIGRTMIN fi } trap parti SIGRTMIN while [ "$NUMPROC" −gt 0 ]; do parti; let "NUMPROC−−" done wait trap − SIGRTMIN exit $? : >) following to end of each script tracked. date>> $SAVE_FILE echo $0>> $SAVE_FILE echo>> $SAVE_FILE
#Date and time. #Script name. #Blank line as separator.
# Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc # (something like ~/.scripts−run)
• The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning? file=data.txt title="***This is the title line of data text file***" echo $title | cat − $file >$file.new # "cat −" concatenates stdout to $file. # End result is #+ to write a new file with $title appended at *beginning*.
Of course, sed can also do this. • A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or even a Makefile. It can be invoked as an external shell command in a C program using the system() call, i.e., system("script_name");. • Put together files containing your favorite and most useful definitions and functions. As necessary, "include" one or more of these "library files" in scripts with either the dot (.) or source command. # SCRIPT LIBRARY # −−−−−− −−−−−−− # Note: # No "#!" here. # No "live code" either.
# Useful variable definitions ROOT_UID=0 E_NOTROOT=101 MAXRETVAL=255 SUCCESS=0 FAILURE=−1
# Root has $UID 0. # Not root user error. # Maximum (positive) return value of a function.
# Functions
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Advanced Bash−Scripting Guide Usage () { if [ −z "$1" ] then msg=filename else msg=$@ fi
# "Usage:" message. # No arg passed.
echo "Usage: `basename $0` "$msg"" }
Check_if_root () # Check if root running script. { # From "ex39.sh" example. if [ "$UID" −ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi }
CreateTempfileName () # Creates a "unique" temp filename. { # From "ex51.sh" example. prefix=temp suffix=`eval date +%s` Tempfilename=$prefix.$suffix }
isalpha2 () # Tests whether *entire string* is alphabetic. { # From "isalpha.sh" example. [ $# −eq 1 ] || return $FAILURE case $1 in *[!a−zA−Z]*|"") return $FAILURE;; *) return $SUCCESS;; esac # Thanks, S.C. }
abs () { E_ARGERR=−999999
# Absolute value. # Caution: Max return value = 255.
if [ −z "$1" ] then return $E_ARGERR fi
# Need arg passed.
if [ "$1" −ge 0 ] then absval=$1 else let "absval = (( 0 − $1 ))" fi
# # # # #
# Obvious error value returned.
If non−negative, stays as−is. Otherwise, change sign.
return $absval }
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Advanced Bash−Scripting Guide tolower () {
# Converts string(s) passed as argument(s) #+ to lowercase.
if [ −z "$1" ] then echo "(null)" return fi
# #+ #+ #+
If no argument(s) passed, send error message (C−style void−pointer error message) and return from function.
echo "$@" | tr A−Z a−z # Translate all passed arguments ($@). return # Use command substitution to set a variable to function output. # For example: # oldvar="A seT of miXed−caSe LEtTerS" # newvar=`tolower "$oldvar"` # echo "$newvar" # a set of mixed−case letters # # Exercise: Rewrite this function to change lowercase passed argument(s) # to uppercase ... toupper() [easy]. }
• Use special−purpose comment headers to increase clarity and legibility in scripts. ## Caution. rm −rf *.zzy
#+ # #+ #+
## The "−rf" options to "rm" are very dangerous, ##+ especially with wildcards.
Line continuation. This is line 1 of a multi−line comment, and this is the final line.
#* Note. #o List item. #> Another point of view. while [ "$var1" != "end" ]
#> while test "$var1" != "end"
• A particularly clever use of if−test constructs is commenting out blocks of code. #!/bin/bash COMMENT_BLOCK= # Try setting the above variable to something or other #+ for an unpleasant surprise. if [ $COMMENT_BLOCK ]; then Comment block −− ================================= This is a comment line. This is another comment line. This is yet another comment line. ================================= echo "This will not echo." Comment blocks are error−free! Whee!
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Advanced Bash−Scripting Guide fi echo "No more comments, please." exit 0
Compare this with using here documents to comment out code blocks. • Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be treated as an integer. #!/bin/bash SUCCESS=0 E_BADINPUT=65 test "$1" −ne 0 −o "$1" −eq 0 2>/dev/null # An integer is either equal to 0 or not equal to 0. # 2>/dev/null suppresses error message. if [ $? −ne "$SUCCESS" ] then echo "Usage: `basename $0` integer−input" exit $E_BADINPUT fi let "sum = $1 + 25" echo "Sum = $sum"
# Would give error if $1 not integer.
# Any variable, not just a command line parameter, can be tested this way. exit 0
• The 0 − 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to stdout (usually with echo) the "return value," and assign this to a variable. This is actually a variant of command substitution. Example 34−11. Return value trickery #!/bin/bash # multiplication.sh multiply () {
# Multiplies params passed. # Will accept a variable number of args.
local product=1 until [ −z "$1" ] do let "product *= $1" shift done
# Until uses up arguments passed...
echo $product
# Will not echo to stdout, #+ since this will be assigned to a variable.
} mult1=15383; mult2=25211 val1=`multiply $mult1 $mult2` echo "$mult1 X $mult2 = $val1"
# 387820813
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Advanced Bash−Scripting Guide mult1=25; mult2=5; mult3=20 val2=`multiply $mult1 $mult2 $mult3` echo "$mult1 X $mult2 X $mult3 = $val2" # 2500 mult1=188; mult2=37; mult3=25; mult4=47 val3=`multiply $mult1 $mult2 $mult3 $mult4` echo "$mult1 X $mult2 X $mult3 X mult4 = $val3" # 8173300 exit 0
The same technique also works for alphanumeric strings. This means that a function can "return" a non−numeric value. capitalize_ichar () {
# Capitalizes initial character #+ of argument string(s) passed.
string0="$@"
# Accepts multiple arguments.
firstchar=${string0:0:1} string1=${string0:1}
# First character. # Rest of string(s).
FirstChar=`echo "$firstchar" | tr a−z A−Z` # Capitalize first character. echo "$FirstChar$string1"
# Output to stdout.
} newstring=`capitalize_ichar "each sentence should start with a capital letter."` echo "$newstring" # Each sentence should start with a capital letter.
It is even possible for a function to "return" multiple values with this method.
Example 34−12. Even more return value trickery #!/bin/bash # sum−product.sh # A function may "return" more than one value. sum_and_product () # Calculates both sum and product of passed args. { echo $(( $1 + $2 )) $(( $1 * $2 )) # Echoes to stdout each calculated value, separated by space. } echo echo "Enter first number " read first echo echo "Enter second number " read second echo retval=`sum_and_product $first $second`
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# Assigns output of function.
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Advanced Bash−Scripting Guide sum=`echo "$retval" | awk '{print $1}'` product=`echo "$retval" | awk '{print $2}'`
# Assigns first field. # Assigns second field.
echo "$first + $second = $sum" echo "$first * $second = $product" echo exit 0
• Next in our bag of trick are techniques for passing an array to a function, then "returning" an array back to the main body of the script. Passing an array involves loading the space−separated elements of the array into a variable with command substitution. Getting an array back as the "return value" from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the ( ... ) operator to assign it to an array.
Example 34−13. Passing and returning arrays #!/bin/bash # array−function.sh: Passing an array to a function and... # "returning" an array from a function
Pass_Array () { local passed_array # Local variable. passed_array=( `echo "$1"` ) echo "${passed_array[@]}" # List all the elements of the new array #+ declared and set within the function. }
original_array=( element1 element2 element3 element4 element5 ) echo echo "original_array = ${original_array[@]}" # List all elements of original array.
# This is the trick that permits passing an array to a function. # ********************************** argument=`echo ${original_array[@]}` # ********************************** # Pack a variable #+ with all the space−separated elements of the original array. # # Note that attempting to just pass the array itself will not work.
# This is the trick that allows grabbing an array as a "return value". # ***************************************** returned_array=( `Pass_Array "$argument"` ) # ***************************************** # Assign 'echoed' output of function to array variable. echo "returned_array = ${returned_array[@]}"
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Advanced Bash−Scripting Guide echo "=============================================================" # Now, try it again, #+ attempting to access (list) the array from outside the function. Pass_Array "$argument" # The function itself lists the array, but... #+ accessing the array from outside the function is forbidden. echo "Passed array (within function) = ${passed_array[@]}" # NULL VALUE since this is a variable local to the function. echo exit 0
For a more elaborate example of passing arrays to functions, see Example A−11. • Using the double parentheses construct, it is possible to use C−like syntax for setting and incrementing variables and in for and while loops. See Example 10−12 and Example 10−17. • Setting the path and umask at the beginning of a script makes it more "portable" −− more likely to run on a "foreign" machine whose user may have bollixed up the $PATH and umask. #!/bin/bash PATH=/bin:/usr/bin:/usr/local/bin ; export PATH umask 022 # Thanks to Ian D. Allen, for this tip.
• A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep. # From "wstrings.sh" example. wlist=`strings "$1" | tr A−Z a−z | tr '[:space:]' Z | \ tr −cs '[:alpha:]' Z | tr −s '\173−\377' Z | tr Z ' '`
Example 34−14. Fun with anagrams #!/bin/bash # agram.sh: Playing games with anagrams. # Find anagrams of... LETTERSET=etaoinshrdlu anagram "$LETTERSET" | grep '.......' | grep '^is' | grep −v 's$' | grep −v 'ed$' # Possible to add many
# Find all anagrams of the letterset... # With at least 7 letters, # starting with 'is' # no plurals # no past tense verbs combinations of conditions.
# Uses "anagram" utility #+ that is part of the author's "yawl" word list package. # http://ibiblio.org/pub/Linux/libs/yawl−0.3.tar.gz exit 0
# End of code.
bash$ sh agram.sh islander isolate
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Advanced Bash−Scripting Guide isolead isotheral
See also Example 28−3, Example 12−21, and Example A−10. • Use "anonymous here documents" to comment out blocks of code, to save having to individually comment out each line with a #. See Example 17−11. • Running a script on a machine that relies on a command that might not be installed is dangerous. Use whatis to avoid potential problems with this. CMD=command1 PlanB=command2
# First choice. # Fallback option.
command_test=$(whatis "$CMD" | grep 'nothing appropriate') # If 'command1' not found on system , 'whatis' will return #+ "command1: nothing appropriate."
if [[ −z "$command_test" ]] then $CMD option1 option2 else $PlanB fi
# Check whether command present. # Run command1 with options. # Otherwise, #+ run command2.
• An if−grep test may not return expected results in an error case, when text is output to stderr, rather that stdout. if ls −l nonexistent_filename | grep −q 'No such file or directory' then echo "File \"nonexistent_filename\" does not exist." fi
Redirecting stderr to stdout fixes this. if ls −l nonexistent_filename 2>&1 | grep −q 'No such file or directory' # ^^^^ then echo "File \"nonexistent_filename\" does not exist." fi # Thanks, Chris Martin, for pointing this out.
• The run−parts command is handy for running a set of command scripts in sequence, particularly in combination with cron or at. • It would be nice to be able to invoke X−Windows widgets from a shell script. There happen to exist several packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to be maintained. Fortunately, it is still possible to obtain widtools here. The widtools (widget tools) package requires the XForms library to be installed. Additionally, the Makefile needs some judicious editing before the package will build on a typical Linux system. Finally, three of the six widgets offered do not work (and, in fact, segfault).
The dialog family of tools offers a method of calling "dialog" widgets from a shell script. The original dialog utility works in a text console, but its successors, gdialog, Xdialog, and kdialog use X−Windows−based widget sets.
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Advanced Bash−Scripting Guide #!/bin/bash # dialog.sh: Using 'gdialog' widgets. # Must have 'gdialog' installed on your system to run this script. # This script was inspired by the following article. # "Scripting for X Productivity," by Marco Fioretti, # LINUX JOURNAL, Issue 113, September 2003, pp. 86−9. # Thank you, all you good people at LJ.
# Input error in dialog box. E_INPUT=65 # Dimensions of display, input widgets. HEIGHT=50 WIDTH=60 # Output file name (constructed out of script name). OUTFILE=$0.output # Display this script in a text widget. gdialog −−title "Displaying: $0" −−textbox $0 $HEIGHT $WIDTH
# Now, we'll try saving input in a file. echo −n "VARIABLE=\"" > $OUTFILE # Quote it, in case of whitespace #+ in the input. gdialog −−title "User Input" −−inputbox "Enter variable, please:" \ $HEIGHT $WIDTH 2>> $OUTFILE
if [ "$?" −eq 0 ] # It's good practice to check exit status. then echo "Executed \"dialog box\" without errors." else echo "Error(s) in \"dialog box\" execution." # Or, clicked on "Cancel", instead of "OK" button. rm $OUTFILE exit $E_INPUT fi
echo −n "\"" >> $OUTFILE # End quotes on saved variable. # This command stuck down here in order not to mess up #+ exit status, above.
# Now, we'll retrieve and display the saved variable. . $OUTFILE # 'Source' the saved file. echo "The variable input in the \"input box\" was: "$VARIABLE"" rm $OUTFILE
# Clean up by removing the temp file. # Some applications may need to retain this file.
exit 0
For other methods of scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk−Perl (Perl with Gtk extensions), or PyQt (Python with Qt extensions).
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34.8. Security Issues A brief warning about script security is appropriate. A shell script may contain a worm, trojan, or even a virus. For that reason, never run as root a script (or permit it to be inserted into the system startup scripts in /etc/rc.d) unless you have obtained said script from a trusted source or you have carefully analyzed it to make certain it does nothing harmful. Various researchers at Bell Labs and other sites, including M. Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the implications of shell script viruses. They conclude that it is all to easy for even a novice, a "script kiddie", to write one. [67] Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your system from being hacked or damaged.
34.9. Portability Issues This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of sh and ksh will find much of value here. As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX 1003.2 standard. Invoking Bash with the −−posix option or inserting a set −o posix at the head of a script causes Bash to conform very closely to this standard. Another alternative is to use a #!/bin/sh
header in the script, rather than #!/bin/bash
Of course, /bin/sh is a link to /bin/bash in Linux and certain other flavors of UNIX. Most Bash scripts will run as−is under ksh, and vice−versa, since Chet Ramey has been busily porting ksh features to the latest versions of Bash. On a commercial Unix machine, scripts using GNU−specific features of standard commands may not work. This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their proprietary counterparts even on "big−iron" Unix. Caldera's release of the source to many of the original Unix utilities has accelerated the trend. Bash has certain features that the traditional Bourne shell lacks. Among these are: • Certain extended invocation options • Command substitution using $( ) notation • Certain string manipulation operations • Process substitution • Bash−specific builtins See the Bash F.A.Q. for a complete listing.
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34.10. Shell Scripting Under Windows Even users running that other OS can run Unix−like shell scripts, and therefore benefit from many of the lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates add shell scripting capabilities to Windows.
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Chapter 35. Bash, version 2 The current version of Bash, the one you have running on your machine, is actually version 2.XX.Y. bash$ echo $BASH_VERSION 2.05.b.0(1)−release
This update of the classic Bash scripting language added array variables, [68] string and parameter expansion, and a better method of indirect variable references, among other features. Example 35−1. String expansion #!/bin/bash # String expansion. # Introduced with version 2 of Bash. # Strings of the form $'xxx' #+ have the standard escaped characters interpreted. echo $'Ringing bell 3 times \a \a \a' # May only ring once with certain terminals. echo $'Three form feeds \f \f \f' echo $'10 newlines \n\n\n\n\n\n\n\n\n\n' echo $'\102\141\163\150' # Bash # Octal equivalent of characters. exit 0
Example 35−2. Indirect variable references − the new way #!/bin/bash # Indirect variable referencing. # This has a few of the attributes of references in C++.
a=letter_of_alphabet letter_of_alphabet=z echo "a = $a"
# Direct reference.
echo "Now a = ${!a}" # Indirect reference. # The ${!variable} notation is greatly superior to the old "eval var1=\$$var2" echo t=table_cell_3 table_cell_3=24 echo "t = ${!t}" table_cell_3=387 echo "Value of t changed to ${!t}"
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# t = 24 # 387
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Advanced Bash−Scripting Guide # This is useful for referencing members of an array or table, #+ or for simulating a multi−dimensional array. # An indexing option would have been nice (sigh). exit 0
Example 35−3. Simple database application, using indirect variable referencing #!/bin/bash # resistor−inventory.sh # Simple database application using indirect variable referencing. # ============================================================== # # Data B1723_value=470 B1723_powerdissip=.25 B1723_colorcode="yellow−violet−brown" B1723_loc=173 B1723_inventory=78
# # # # #
Ohms Watts Color bands Where they are How many
B1724_value=1000 B1724_powerdissip=.25 B1724_colorcode="brown−black−red" B1724_loc=24N B1724_inventory=243 B1725_value=10000 B1725_powerdissip=.25 B1725_colorcode="brown−black−orange" B1725_loc=24N B1725_inventory=89 # ============================================================== #
echo PS3='Enter catalog number: ' echo select catalog_number in "B1723" "B1724" "B1725" do Inv=${catalog_number}_inventory Val=${catalog_number}_value Pdissip=${catalog_number}_powerdissip Loc=${catalog_number}_loc Ccode=${catalog_number}_colorcode echo echo echo echo echo
"Catalog number $catalog_number:" "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt] resistors in stock." "These are located in bin # ${!Loc}." "Their color code is \"${!Ccode}\"."
break done echo; echo
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Advanced Bash−Scripting Guide # Exercises: # −−−−−−−−− # 1) Rewrite this script to read its data from an external file. # 2) Rewrite this script to use arrays, #+ rather than indirect variable referencing. # Which method is more straightforward and intuitive?
# Notes: # −−−−− # Shell scripts are inappropriate for anything except the most simple #+ database applications, and even then it involves workarounds and kludges. # Much better is to use a language with native support for data structures, #+ such as C++ or Java (or even Perl). exit 0
Example 35−4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards #!/bin/bash # May need to be invoked with
#!/bin/bash2
on older machines.
# Cards: # deals four random hands from a deck of cards. UNPICKED=0 PICKED=1 DUPE_CARD=99 LOWER_LIMIT=0 UPPER_LIMIT=51 CARDS_IN_SUIT=13 CARDS=52 declare −a Deck declare −a Suits declare −a Cards # It would have been easier and more intuitive # with a single, 3−dimensional array. # Perhaps a future version of Bash will support multidimensional arrays.
initialize_Deck () { i=$LOWER_LIMIT until [ "$i" −gt $UPPER_LIMIT ] do Deck[i]=$UNPICKED # Set each card of "Deck" as unpicked. let "i += 1" done echo } initialize_Suits () { Suits[0]=C #Clubs Suits[1]=D #Diamonds
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Advanced Bash−Scripting Guide Suits[2]=H #Hearts Suits[3]=S #Spades } initialize_Cards () { Cards=(2 3 4 5 6 7 8 9 10 J Q K A) # Alternate method of initializing an array. } pick_a_card () { card_number=$RANDOM let "card_number %= $CARDS" if [ "${Deck[card_number]}" −eq $UNPICKED ] then Deck[card_number]=$PICKED return $card_number else return $DUPE_CARD fi } parse_card () { number=$1 let "suit_number = number / CARDS_IN_SUIT" suit=${Suits[suit_number]} echo −n "$suit−" let "card_no = number % CARDS_IN_SUIT" Card=${Cards[card_no]} printf %−4s $Card # Print cards in neat columns. } seed_random () # Seed random number generator. { seed=`eval date +%s` let "seed %= 32766" RANDOM=$seed } deal_cards () { echo cards_picked=0 while [ "$cards_picked" −le $UPPER_LIMIT ] do pick_a_card t=$? if [ "$t" −ne $DUPE_CARD ] then parse_card $t u=$cards_picked+1 # Change back to 1−based indexing (temporarily). let "u %= $CARDS_IN_SUIT" if [ "$u" −eq 0 ] # Nested if/then condition test. then echo
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Advanced Bash−Scripting Guide echo fi # Separate hands. let "cards_picked += 1" fi done echo return 0 }
# Structured programming: # entire program logic modularized in functions. #================ seed_random initialize_Deck initialize_Suits initialize_Cards deal_cards exit 0 #================
# Exercise 1: # Add comments to thoroughly document this script. # Exercise 2: # Revise the script to print out each hand sorted in suits. # You may add other bells and whistles if you like. # Exercise 3: # Simplify and streamline the logic of the script.
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Chapter 36. Endnotes 36.1. Author's Note How did I come to write a Bash scripting book? It's a strange tale. It seems that a couple of years back, I needed to learn shell scripting −− and what better way to do that than to read a good book on the subject? I was looking to buy a tutorial and reference covering all aspects of the subject. I was looking for a book that would take difficult concepts, turn them inside out, and explain them in excruciating detail, with well−commented examples. [69] In fact, I was looking for this very book, or something much like it. Unfortunately, it didn't exist, and if I wanted it, I'd have to write it. And so, here we are, folks. This reminds me of the apocryphal story about the mad professor. Crazy as a loon, the fellow was. At the sight of a book, any book −− at the library, at a bookstore, anywhere −− he would become totally obsessed with the idea that he could have written it, should have written it, and done a better job of it to boot. He would thereupon rush home and proceed to do just that, write a book with the very same title. When he died some years later, he allegedly had several thousand books to his credit, probably putting even Asimov to shame. The books might not have been any good −− who knows −− but does that really matter? Here's a fellow who lived his dream, even if he was obsessed by it, driven by it, and I can't help admiring the old coot...
36.2. About the Author Who is this guy anyhow? The author claims no credentials or special qualifications, other than a compulsion to write. [70] This book is somewhat of a departure from his other major work, HOW−2 Meet Women: The Shy Man's Guide to Relationships. He has also written the Software−Building HOWTO. Lately, he has been trying his hand at short fiction. A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has emitted a few software truffles, including the cruft one−time pad encryption utility, the mcalc mortgage calculator, the judge Scrabble® adjudicator, and the yawl word gaming list package. He got his start in programming using FORTRAN IV on a CDC 3800, but is not the least bit nostalgic for those days. Living in a secluded desert community with wife and dog, he cherishes human frailty.
36.3. Where to Go For Help The author will usually, if not too busy (and in a good mood), answer general scripting questions. However, if you have a problem getting a specific script to work, you would be well advised to post to the comp.os.unix.shell Usenet newsgroup.
36.4. Tools Used to Produce This Book 36.4.1. Hardware A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running Red Hat 7.1/7.3. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet. Chapter 36. Endnotes
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36.4.2. Software and Printware i. Bram Moolenaar's powerful SGML−aware vim text editor. ii. OpenJade, a DSSSL rendering engine for converting SGML documents into other formats. iii. Norman Walsh's DSSSL stylesheets. iv. DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1−56592−580−7). This is the standard reference for anyone attempting to write a document in Docbook SGML format.
36.5. Credits Community participation made this project possible. The author gratefully acknowledges that writing this book would have been an impossible task without help and feedback from all you people out there. Philippe Martin translated this document into DocBook/SGML. While not on the job at a small French company as a software developer, he enjoys working on GNU/Linux documentation and software, reading literature, playing music, and for his peace of mind making merry with friends. You may run across him somewhere in France or in the Basque Country, or email him at
[email protected]. Philippe Martin also pointed out that positional parameters past $9 are possible using {bracket} notation, see Example 4−5. Stephane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he has, in effect, taken on the role of editor for this document. Merci beaucoup! Paulo Marcel Coelho Aragao offered many corrections, both major and minor, and contributed quite a number of helpful suggestions. I would like to especially thank Patrick Callahan, Mike Novak, and Pal Domokos for catching bugs, pointing out ambiguities, and for suggesting clarifications and changes. Their lively discussion of shell scripting and general documentation issues inspired me to try to make this document more readable. I'm grateful to Jim Van Zandt for pointing out errors and omissions in version 0.2 of this document. He also contributed an instructive example script. Many thanks to Jordi Sanfeliu for giving permission to use his fine tree script (Example A−18), and to Rick Boivie for revising it. Likewise, thanks to Michel Charpentier for permission to use his dc factoring script (Example 12−41). Kudos to Noah Friedman for permission to use his string function script (Example A−19). Emmanuel Rouat suggested corrections and additions on command substitution and aliases. He also contributed a very nice sample .bashrc file (Appendix J). Heiner Steven kindly gave permission to use his base conversion script, Example 12−37. He also made a number of corrections and many helpful suggestions. Special thanks.
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Advanced Bash−Scripting Guide Rick Boivie contributed the delightfully recursive pb.sh script (Example 34−7), revised the tree.sh script (Example A−18), and suggested performance improvements for the monthlypmt.sh script (Example 12−36). Florian Wisser enlightened me on some of the fine points of testing strings (see Example 7−6), and on other matters. Oleg Philon sent suggestions concerning cut and pidof. Michael Zick extended the empty array example to demonstrate some surprising array properties. He also provided other examples of this. Marc−Jano Knopp sent corrections on DOS batch files. Hyun Jin Cha found several typos in the document in the process of doing a Korean translation. Thanks for pointing these out. Andreas Abraham sent in a long list of typographical errors and other corrections. Special thanks! Others contributing scripts, making helpful suggestions, and pointing out errors were Gabor Kiss, Leopold Toetsch, Peter Tillier, Marcus Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell, Jess Thrysoee, Adam Lazur, Bram Moolenaar, Baris Cicek, Greg Keraunen, Keith Matthews, Sandro Magi, Albert Reiner, Dim Segebart, Rory Winston, Lee Bigelow, Wayne Pollock, "jipe," "Mark," Emilio Conti, Ian. D. Allen, Arun Giridhar, Dennis Leeuw, Dan Jacobson, Aurelio Marinho Jargas, Edward Scholtz, Jean Helou, Chris Martin, Lee Maschmeyer, Bruno Haible, Sebastien Godard, Bjön Eriksson, "nyal," John MacDonald, Joshua Tschida, Troy Engel, Manfred Schwarb, Amit Singh, Bill Gradwohl, David Lombard, Jason Parker, Bruce W. Clare, William Park, Vernia Damiano, and David Lawyer (himself an author of four HOWTOs). My gratitude to Chet Ramey and Brian Fox for writing Bash, an elegant and powerful scripting tool. Very special thanks to the hard−working volunteers at the Linux Documentation Project. The LDP hosts a repository of Linux knowledge and lore, and has, to a large extent, enabled the publication of this book. Thanks and appreciation to IBM, Novell, Red Hat, the Free Software Foundation, and all the good people fighting the good fight to keep Open Source software free and open. Thanks most of all to my wife, Anita, for her encouragement and emotional support.
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Bibliography Edited by Peter Denning, Computers Under Attack: Intruders, Worms, and Viruses, ACM Press, 1990, 0−201−53067−8. This compendium contains a couple of articles on shell script viruses. *
Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997, 1−156592−225−5. To unfold the full power of shell scripting, you need at least a passing familiarity with sed and awk. This is the standard tutorial. It includes an excellent introduction to "regular expressions". Read this book. *
Jeffrey Friedl, Mastering Regular Expressions, O'Reilly and Associates, 2002, 0−596−00289−0. The best, all−around reference on Regular Expressions. *
Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly and Associates, 2002, 0−596−00343−9. This excellent sys admin manual has a decent introduction to shell scripting for sys administrators and does a nice job of explaining the startup and initialization scripts. The long overdue third edition of this classic has finally been released. *
Stephen Kochan and Patrick Woods, Unix Shell Programming, Hayden, 1990, 067248448X. The standard reference, though a bit dated by now. *
Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox Press, 1996, 1874416680. Good in−depth coverage of various programming languages available for Linux, including a fairly strong chapter on shell scripting. *
Bibliography
405
Advanced Bash−Scripting Guide Herbert Mayer, Advanced C Programming on the IBM PC, Windcrest Books, 1989, 0830693637. Excellent coverage of algorithms and general programming practices. *
David Medinets, Unix Shell Programming Tools, McGraw−Hill, 1999, 0070397333. Good info on shell scripting, with examples, and a short intro to Tcl and Perl. *
Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998, 1−56592−347−2. This is a valiant effort at a decent shell primer, but somewhat deficient in coverage on programming topics and lacking sufficient examples. *
Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X. A very handy pocket reference, despite lacking coverage of Bash−specific features. *
Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 2nd edition, O'Reilly and Associates, Random House, 1997, 1−56592−260−3. Contains a couple of sections of very informative in−depth articles on shell programming, but falls short of being a tutorial. It reproduces much of the regular expressions tutorial from the Dougherty and Robbins book, above. *
Clifford Pickover, Computers, Pattern, Chaos, and Beauty, St. Martin's Press, 1990, 0−312−04123−3. A treasure trove of ideas and recipes for computer−based exploration of mathematical oddities. *
George Polya, How To Solve It, Princeton University Press, 1973, 0−691−02356−5. The classic tutorial on problem solving methods (i.e., algorithms).
Bibliography
406
Advanced Bash−Scripting Guide *
Chet Ramey and Brian Fox, The GNU Bash Reference Manual, Network Theory Ltd, 2003, 0−9541617−7−7. This manual is the definitive reference for GNU Bash. The authors of this manual, Chet Ramey and Brian Fox, are the original developers of GNU Bash. For each copy sold the publisher donates $1 to the Free Software Foundation.
Arnold Robbins, Bash Reference Card, SSC, 1998, 1−58731−010−5. Excellent Bash pocket reference (don't leave home without it). A bargain at $4.95, but also available for free download on−line in pdf format. *
Arnold Robbins, Effective Awk Programming, Free Software Foundation / O'Reilly and Associates, 2000, 1−882114−26−4. The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk documentation, and printed copies of the latest version are available from O'Reilly and Associates. This book has served as an inspiration for the author of this document. *
Bill Rosenblatt, Learning the Korn Shell, O'Reilly and Associates, 1993, 1−56592−054−6. This well−written book contains some excellent pointers on shell scripting. *
Paul Sheer, LINUX: Rute User's Tutorial and Exposition, 1st edition, , 2002, 0−13−033351−4. Very detailed and readable introduction to Linux system administration. The book is available in print, or on−line. *
Ellen Siever and the staff of O'Reilly and Associates, Linux in a Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1−56592−585−8. The all−around best Linux command reference, even has a Bash section. * Bibliography
407
Advanced Bash−Scripting Guide The UNIX CD Bookshelf, 3rd edition, O'Reilly and Associates, 2003, 0−596−00392−7. An array of seven UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $130. Buy this one, even if it means going into debt and not paying the rent. *
The O'Reilly books on Perl. (Actually, any O'Reilly books.) −−−
Fioretti, Marco, "Scripting for X Productivity," LINUX JOURNAL, Issue 113, September, 2003, pp. 86−9.
Ben Okopnik's well−written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux Gazette , and his explanation of "The Deep, Dark Secrets of Bash" in issue 56.
Chet Ramey's bash − The GNU Shell, a two−part series published in issues 3 and 4 of the Linux Journal, July−August 1994.
Mike G's Bash−Programming−Intro HOWTO.
Richard's Unix Scripting Universe.
Chet Ramey's Bash F.A.Q.
Ed Schaefer's Shell Corner in Unix Review.
Example shell scripts at Lucc's Shell Scripts .
Example shell scripts at SHELLdorado .
Example shell scripts at Noah Friedman's script site.
Steve Parker's Shell Programming Stuff.
Example shell scripts at SourceForge Snippet Library − shell scrips.
Bibliography
408
Advanced Bash−Scripting Guide Giles Orr's Bash−Prompt HOWTO.
Very nice sed, awk, and regular expression tutorials at The UNIX Grymoire.
Eric Pement's sed resources page.
The GNU gawk reference manual (gawk is the extended GNU version of awk available on Linux and BSD systems).
Trent Fisher's groff tutorial.
Mark Komarinski's Printing−Usage HOWTO.
There is some nice material on I/O redirection in chapter 10 of the textutils documentation at the University of Alberta site.
Rick Hohensee has written the osimpa i386 assembler entirely as Bash scripts.
Aurelio Marinho Jargas has written a Regular expression wizard. He has also written an informative book on Regular Expressions, in Portuguese.
Ben Tomkins has created the Bash Navigator directory management tool.
William Park is working on a project to incorporate certain Awk and Python features into Bash.
Rocky Bernstein is in the process of developing a "full−fledged" debugger for Bash. −−−
The excellent "Bash Reference Manual", by Chet Ramey and Brian Fox, distributed as part of the "bash−2−doc" package (available as an rpm). See especially the instructive example scripts in this package.
The comp.os.unix.shell newsgroup.
The comp.os.unix.shell FAQ and its mirror site.
Bibliography
409
Advanced Bash−Scripting Guide Assorted comp.os.unix FAQs.
The manpages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, bc, xargs. The texinfo documentation on bash, dd, m4, gawk, and sed.
Bibliography
410
Appendix A. Contributed Scripts These scripts, while not fitting into the text of this document, do illustrate some interesting shell programming techniques. They are useful, too. Have fun analyzing and running them.
Example A−1. manview: Viewing formatted manpages #!/bin/bash # manview.sh: Formats the source of a man page for viewing. # This script is useful when writing man page source. # It lets you look at the intermediate results on the fly #+ while working on it. E_WRONGARGS=65 if [ −z "$1" ] then echo "Usage: `basename $0` filename" exit $E_WRONGARGS fi # −−−−−−−−−−−−−−−−−−−−−−−−−−− groff −Tascii −man $1 | less # From the man page for groff. # −−−−−−−−−−−−−−−−−−−−−−−−−−− # If the man page includes tables and/or equations, #+ then the above code will barf. # The following line can handle such cases. # # gtbl < "$1" | geqn −Tlatin1 | groff −Tlatin1 −mtty−char −man # # Thanks, S.C. exit 0
Example A−2. mailformat: Formatting an e−mail message #!/bin/bash # mail−format.sh: Format e−mail messages. # Gets rid of carets, tabs, also fold excessively long lines. # ================================================================= # Standard Check for Script Argument(s) ARGS=1 E_BADARGS=65 E_NOFILE=66 if [ $# −ne $ARGS ] # Correct number of arguments passed to script? then echo "Usage: `basename $0` filename" exit $E_BADARGS fi
Appendix A. Contributed Scripts
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Advanced Bash−Scripting Guide if [ −f "$1" ] # Check if file exists. then file_name=$1 else echo "File \"$1\" does not exist." exit $E_NOFILE fi # ================================================================= MAXWIDTH=70
# Width to fold long lines to.
# Delete carets and tabs at beginning of lines, #+ then fold lines to $MAXWIDTH characters. sed ' s/^>// s/^ *>// s/^ *// s/ *// ' $1 | fold −s −−width=$MAXWIDTH # −s option to "fold" breaks lines at whitespace, if possible. # #+ # # #+
This script was inspired by an article in a well−known trade journal extolling a 164K Windows utility with similar functionality. An nice set of text processing utilities and an efficient scripting language provide an alternative to bloated executables.
exit 0
Example A−3. rn: A simple−minded file rename utility This script is a modification of Example 12−18. #! /bin/bash # # Very simpleminded filename "rename" utility (based on "lowercase.sh"). # # The "ren" utility, by Vladimir Lanin (
[email protected]), #+ does a much better job of this.
ARGS=2 E_BADARGS=65 ONE=1
# For getting singular/plural right (see below).
if [ $# −ne "$ARGS" ] then echo "Usage: `basename $0` old−pattern new−pattern" # As in "rn gif jpg", which renames all gif files in working directory to jpg. exit $E_BADARGS fi number=0
# Keeps track of how many files actually renamed.
for filename in *$1* do if [ −f "$filename" ] then
#Traverse all matching files in directory. # If finds match...
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Advanced Bash−Scripting Guide fname=`basename $filename` n=`echo $fname | sed −e "s/$1/$2/"` mv $fname $n let "number += 1" fi done
# Strip off path. # Substitute new for old in filename. # Rename.
if [ "$number" −eq "$ONE" ] then echo "$number file renamed." else echo "$number files renamed." fi
# For correct grammar.
exit 0
# Exercises: # −−−−−−−−− # What type of files will this not work on? # How can this be fixed? # # Rewrite this script to process all the files in a directory #+ containing spaces in their names, and to rename them, #+ substituting an underscore for each space.
Example A−4. blank−rename: renames filenames containing blanks This is an even simpler−minded version of previous script. #! /bin/bash # blank−rename.sh # # Substitutes underscores for blanks in all the filenames in a directory. ONE=1 number=0 FOUND=0
# For getting singular/plural right (see below). # Keeps track of how many files actually renamed. # Successful return value.
for filename in * #Traverse all files in directory. do echo "$filename" | grep −q " " # Check whether filename if [ $? −eq $FOUND ] #+ contains space(s). then fname=$filename # Strip off path. n=`echo $fname | sed −e "s/ /_/g"` # Substitute underscore for blank. mv "$fname" "$n" # Do the actual renaming. let "number += 1" fi done if [ "$number" −eq "$ONE" ] then echo "$number file renamed." else echo "$number files renamed." fi
# For correct grammar.
exit 0
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Advanced Bash−Scripting Guide Example A−5. encryptedpw: Uploading to an ftp site, using a locally encrypted password #!/bin/bash # Example "ex72.sh" modified to use encrypted password. # Note that this is still rather insecure, #+ since the decrypted password is sent in the clear. # Use something like "ssh" if this is a concern. E_BADARGS=65 if [ −z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS fi Username=bozo # Change to suit. pword=/home/bozo/secret/password_encrypted.file # File containing encrypted password. Filename=`basename $1`
# Strips pathname out of file name
Server="XXX" Directory="YYY"
# Change above to actual server name & directory.
Password=`cruft If no command line args present, then works on file redirected to stdin. sed −e '1,/^$/d' −e '/^[ ]*$/d' # −−> Delete empty lines and all lines until # −−> first one beginning with white space. else # ==> If command line args present, then work on files named. for i do sed −e '1,/^$/d' −e '/^[ ]*$/d' $i # −−> Ditto, as above. done fi
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Advanced Bash−Scripting Guide # # # #
==> Exercise: Add error checking and other options. ==> ==> Note that the small sed script repeats, except for the arg passed. ==> Does it make sense to embed it in a function? Why or why not?
Example A−14. ftpget: Downloading files via ftp #! /bin/sh # $Id: ftpget,v 1.2 91/05/07 21:15:43 moraes Exp $ # Script to perform batch anonymous ftp. Essentially converts a list of # of command line arguments into input to ftp. # Simple, and quick − written as a companion to ftplist # −h specifies the remote host (default prep.ai.mit.edu) # −d specifies the remote directory to cd to − you can provide a sequence # of −d options − they will be cd'ed to in turn. If the paths are relative, # make sure you get the sequence right. Be careful with relative paths − # there are far too many symlinks nowadays. # (default is the ftp login directory) # −v turns on the verbose option of ftp, and shows all responses from the # ftp server. # −f remotefile[:localfile] gets the remote file into localfile # −m pattern does an mget with the specified pattern. Remember to quote # shell characters. # −c does a local cd to the specified directory # For example, # ftpget −h expo.lcs.mit.edu −d contrib −f xplaces.shar:xplaces.sh \ # −d ../pub/R3/fixes −c ~/fixes −m 'fix*' # will get xplaces.shar from ~ftp/contrib on expo.lcs.mit.edu, and put it in # xplaces.sh in the current working directory, and get all fixes from # ~ftp/pub/R3/fixes and put them in the ~/fixes directory. # Obviously, the sequence of the options is important, since the equivalent # commands are executed by ftp in corresponding order # # Mark Moraes (
[email protected]), Feb 1, 1989 # ==> Angle brackets changed to parens, so Docbook won't get indigestion. #
# ==> These comments added by author of this document. # PATH=/local/bin:/usr/ucb:/usr/bin:/bin # export PATH # ==> Above 2 lines from original script probably superfluous. TMPFILE=/tmp/ftp.$$ # ==> Creates temp file, using process id of script ($$) # ==> to construct filename. SITE=`domainname`.toronto.edu # ==> 'domainname' similar to 'hostname' # ==> May rewrite this to parameterize this for general use. usage="Usage: $0 [−h remotehost] [−d remotedirectory]... [−f remfile:localfile]... \ [−c localdirectory] [−m filepattern] [−v]" ftpflags="−i −n" verbflag= set −f # So we can use globbing in −m set x `getopt vh:d:c:m:f: $*` if [ $? != 0 ]; then echo $usage
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Advanced Bash−Scripting Guide exit 65 fi shift trap 'rm −f ${TMPFILE} ; exit' 0 1 2 3 15 echo "user anonymous ${USER−gnu}@${SITE} > ${TMPFILE}" # ==> Added quotes (recommended in complex echoes). echo binary >> ${TMPFILE} for i in $* # ==> Parse command line args. do case $i in −v) verbflag=−v; echo hash >> ${TMPFILE}; shift;; −h) remhost=$2; shift 2;; −d) echo cd $2 >> ${TMPFILE}; if [ x${verbflag} != x ]; then echo pwd >> ${TMPFILE}; fi; shift 2;; −c) echo lcd $2 >> ${TMPFILE}; shift 2;; −m) echo mget "$2" >> ${TMPFILE}; shift 2;; −f) f1=`expr "$2" : "\([^:]*\).*"`; f2=`expr "$2" : "[^:]*:\(.*\)"`; echo get ${f1} ${f2} >> ${TMPFILE}; shift 2;; −−) shift; break;; esac done if [ $# −ne 0 ]; then echo $usage exit 65 # ==> Changed from "exit 2" to conform with standard. fi if [ x${verbflag} != x ]; then ftpflags="${ftpflags} −v" fi if [ x${remhost} = x ]; then remhost=prep.ai.mit.edu # ==> Rewrite to match your favorite ftp site. fi echo quit >> ${TMPFILE} # ==> All commands saved in tempfile. ftp ${ftpflags} ${remhost} < ${TMPFILE} # ==> Now, tempfile batch processed by ftp. rm −f ${TMPFILE} # ==> Finally, tempfile deleted (you may wish to copy it to a logfile).
# # # #
==> ==> ==> ==>
Exercises: −−−−−−−−− 1) Add error checking. 2) Add bells & whistles.
+ Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution operators discussed in Section 9.3.
Example A−15. password: Generating random 8−character passwords #!/bin/bash # May need to be invoked with
Appendix A. Contributed Scripts
#!/bin/bash2
on older machines.
430
Advanced Bash−Scripting Guide # # Random password generator for Bash 2.x by Antek Sawicki , # who generously gave permission to the document author to use it here. # # ==> Comments added by document author ==>
MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" # ==> Password will consist of alphanumeric characters. LENGTH="8" # ==> May change 'LENGTH' for longer password.
while [ "${n:=1}" −le "$LENGTH" ] # ==> Recall that := is "default substitution" operator. # ==> So, if 'n' has not been initialized, set it to 1. do PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" # ==> Very clever, but tricky. # ==> Starting from the innermost nesting... # ==> ${#MATRIX} returns length of array MATRIX. # ==> $RANDOM%${#MATRIX} returns random number between 1 # ==> and [length of MATRIX] − 1. # # # #
==> ==> ==> ==>
${MATRIX:$(($RANDOM%${#MATRIX})):1} returns expansion of MATRIX at random position, by length 1. See {var:pos:len} parameter substitution in Chapter 9. and the associated examples.
# ==> PASS=... simply pastes this result onto previous PASS (concatenation). # ==> To visualize this more clearly, uncomment the following line # echo "$PASS" # ==> to see PASS being built up, # ==> one character at a time, each iteration of the loop. let n+=1 # ==> Increment 'n' for next pass. done echo "$PASS"
# ==> Or, redirect to a file, as desired.
exit 0
+ James R. Van Zandt contributed this script, which uses named pipes and, in his words, "really exercises quoting and escaping".
Example A−16. fifo: Making daily backups, using named pipes #!/bin/bash # ==> Script by James R. Van Zandt, and used here with his permission. # ==> Comments added by author of this document.
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Advanced Bash−Scripting Guide HERE=`uname −n` # ==> hostname THERE=bilbo echo "starting remote backup to $THERE at `date +%r`" # ==> `date +%r` returns time in 12−hour format, i.e. "08:08:34 PM". # make sure /pipe really is a pipe and not a plain file rm −rf /pipe mkfifo /pipe # ==> Create a "named pipe", named "/pipe". # ==> 'su xyz' runs commands as user "xyz". # ==> 'ssh' invokes secure shell (remote login client). su xyz −c "ssh $THERE \"cat >/home/xyz/backup/${HERE}−daily.tar.gz\" < /pipe"& cd / tar −czf − bin boot dev etc home info lib man root sbin share usr var >/pipe # ==> Uses named pipe, /pipe, to communicate between processes: # ==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe. # ==> The end result is this backs up the main directories, from / on down. # ==> What are the advantages of a "named pipe" in this situation, # ==> as opposed to an "anonymous pipe", with |? # ==> Will an anonymous pipe even work here?
exit 0
+ Stephane Chazelas contributed the following script to demonstrate that generating prime numbers does not require arrays.
Example A−17. Generating prime numbers using the modulo operator #!/bin/bash # primes.sh: Generate prime numbers, without using arrays. # Script contributed by Stephane Chazelas. # This does *not* use the classic "Sieve of Eratosthenes" algorithm, #+ but instead uses the more intuitive method of testing each candidate number #+ for factors (divisors), using the "%" modulo operator.
LIMIT=1000 Primes() { (( n = $1 + 1 )) shift # echo "_n=$n i=$i_"
# Primes 2 − 1000
# Bump to next integer. # Next parameter in list.
if (( n == LIMIT )) then echo $* return fi for i; do # echo "−n=$n i=$i−" (( i * i > n )) && break (( n % i )) && continue
Appendix A. Contributed Scripts
# "i" gets set to "@", previous values of $n. # Optimization. # Sift out non−primes using modulo operator.
432
Advanced Bash−Scripting Guide Primes $n $@ return done
# Recursion inside loop.
Primes $n $@ $n
# Recursion outside loop. # Successively accumulate positional parameters. # "$@" is the accumulating list of primes.
} Primes 1 exit 0 #
Uncomment lines 16 and 24 to help figure out what is going on.
# Compare the speed of this algorithm for generating primes #+ with the Sieve of Eratosthenes (ex68.sh). #
Exercise: Rewrite this script without recursion, for faster execution.
+ This is Rick Boivie's revision of Jordi Sanfeliu's tree script.
Example A−18. tree: Displaying a directory tree #!/bin/bash # tree.sh # # # # # #+
Written by Rick Boivie. Used with permission. This is a revised and simplified version of a script by Jordi Sanfeliu (and patched by Ian Kjos). This script replaces the earlier version used in previous releases of the Advanced Bash Scripting Guide.
# ==> Comments added by the author of this document.
search () { for dir in `echo *` # ==> `echo *` lists all the files in current working directory, #+ ==> without line breaks. # ==> Similar effect to for dir in * # ==> but "dir in `echo *`" will not handle filenames with blanks. do if [ −d "$dir" ] ; then # ==> If it is a directory (−d)... zz=0 # ==> Temp variable, keeping track of directory level. while [ $zz != $1 ] # Keep track of inner nested loop. do echo −n "| " # ==> Display vertical connector symbol, # ==> with 2 spaces & no line feed in order to indent. zz=`expr $zz + 1` # ==> Increment zz. done if [ −L "$dir" ] ; then # ==> If directory is a symbolic link... echo "+−−−$dir" `ls −l $dir | sed 's/^.*'$dir' //'` # ==> Display horiz. connector and list directory name, but... # ==> delete date/time part of long listing.
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Advanced Bash−Scripting Guide else echo "+−−−$dir" # ==> Display horizontal connector symbol... # ==> and print directory name. numdirs=`expr $numdirs + 1` # ==> Increment directory count. if cd "$dir" ; then # ==> If can move to subdirectory... search `expr $1 + 1` # with recursion ;−) # ==> Function calls itself. cd .. fi fi fi done } if [ $# != 0 ] ; then cd $1 # move to indicated directory. #else # stay in current directory fi echo "Initial directory = `pwd`" numdirs=0 search 0 echo "Total directories = $numdirs" exit 0
Noah Friedman gave permission to use his string function script, which essentially reproduces some of the C−library string manipulation functions.
Example A−19. string functions: C−like string functions #!/bin/bash # # # # # #
string.bash −−− bash emulation of string(3) library routines Author: Noah Friedman ==> Used with his kind permission in this document. Created: 1992−07−01 Last modified: 1993−09−29 Public domain
# Conversion to bash v2 syntax done by Chet Ramey # Commentary: # Code: #:docstring strcat: # Usage: strcat s1 s2 # # Strcat appends the value of variable s2 to variable s1. # # Example: # a="foo" # b="bar" # strcat a b # echo $a # => foobar # #:end docstring:
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Advanced Bash−Scripting Guide ###;;;autoload ==> Autoloading of function commented out. function strcat () { local s1_val s2_val s1_val=${!1} # indirect variable expansion s2_val=${!2} eval "$1"=\'"${s1_val}${s2_val}"\' # ==> eval $1='${s1_val}${s2_val}' avoids problems, # ==> if one of the variables contains a single quote. } #:docstring strncat: # Usage: strncat s1 s2 $n # # Line strcat, but strncat appends a maximum of n characters from the value # of variable s2. It copies fewer if the value of variabl s2 is shorter # than n characters. Echoes result on stdout. # # Example: # a=foo # b=barbaz # strncat a b 3 # echo $a # => foobar # #:end docstring: ###;;;autoload function strncat () { local s1="$1" local s2="$2" local −i n="$3" local s1_val s2_val s1_val=${!s1} s2_val=${!s2} if [ ${#s2_val} −gt ${n} ]; then s2_val=${s2_val:0:$n} fi
# ==> indirect variable expansion
# ==> substring extraction
eval "$s1"=\'"${s1_val}${s2_val}"\' # ==> eval $1='${s1_val}${s2_val}' avoids problems, # ==> if one of the variables contains a single quote. } #:docstring strcmp: # Usage: strcmp $s1 $s2 # # Strcmp compares its arguments and returns an integer less than, equal to, # or greater than zero, depending on whether string s1 is lexicographically # less than, equal to, or greater than string s2. #:end docstring: ###;;;autoload function strcmp () { [ "$1" = "$2" ] && return 0
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435
Advanced Bash−Scripting Guide [ "${1}" ' /home/mszick/.xsession−errors /proc/982/fd/13 −> /tmp/tmpfZVVOCs (deleted) /proc/982/fd/7 −> /tmp/kde−mszick/ksycoca /proc/982/fd/8 −> socket:[11586] /proc/982/fd/9 −> pipe:[11588] If that isn't enough to keep your parser guessing, either or both of the path components may be relative: ../Built−Shared −> Built−Static ../linux−2.4.20.tar.bz2 −> ../../../SRCS/linux−2.4.20.tar.bz2 The first character of the 11 (10?) character permissions field: 's' Socket 'd' Directory 'b' Block device 'c' Character device 'l' Symbolic link NOTE: Hard links not marked − test for identical inode numbers on identical filesystems. All information about hard linked files are shared, except for the names and the name's location in the directory system. NOTE: A "Hard link" is known as a "File Alias" on some systems. '−' An undistingushed file Followed by three groups of letters for: User, Group, Others Character 1: '−' Not readable; 'r' Readable Character 2: '−' Not writable; 'w' Writable Character 3, User and Group: Combined execute and special '−' Not Executable, Not Special 'x' Executable, Not Special 's' Executable, Special 'S' Not Executable, Special Character 3, Others: Combined execute and sticky (tacky?) '−' Not Executable, Not Tacky 'x' Executable, Not Tacky 't' Executable, Tacky 'T' Not Executable, Tacky Followed by an access indicator Haven't tested this one, it may be the eleventh character or it may generate another field ' ' No alternate access '+' Alternate access LSfieldsDoc
ListDirectory() { local −a T local −i of=0 # OLD_IFS=$IFS
# Default return in variable # Using BASH default ' \t\n'
case "$#" in 3) case "$1" in −of) of=1 ; shift ;; * ) return 1 ;; esac ;; 2) : ;; # Poor man's "continue"
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440
Advanced Bash−Scripting Guide *) esac
return 1 ;;
# NOTE: the (ls) command is NOT quoted (") T=( $(ls −−inode −−ignore−backups −−almost−all −−directory \ −−full−time −−color=none −−time=status −−sort=none \ −−format=long $1) ) case $of in # Assign T back to the array whose name was passed as $2 0) eval $2=\( \"\$\{T\[@\]\}\" \) ;; # Write T into filename passed as $2 1) echo "${T[@]}" > "$2" ;; esac return 0 } # # # # # Is that string a legal number? # # # # # # # IsNumber "Var" # # # # # There has to be a better way, sigh... IsNumber() { local −i int if [ $# −eq 0 ] then return 1 else (let int=$1) return $? fi }
2>/dev/null # Exit status of the let thread
# # # # # Index Filesystem Directory Information # # # # # # # IndexList "Field−Array−Name" "Index−Array−Name" # or # IndexList −if Field−Array−Filename Index−Array−Name # IndexList −of Field−Array−Name Index−Array−Filename # IndexList −if −of Field−Array−Filename Index−Array−Filename # # # # # : /dev/null fi ## # Check if the mount point exists and create if it doesn't. # if [ ! −e $MOUNTPOINT ]; then mkdir −p $MOUNTPOINT fi ## # Take care of /etc/fstab so mounting is easy. # if ! grep −q "^$SYMLINKDEV" /etc/fstab; then # Add an fstab entry echo −e \ "$SYMLINKDEV\t\t$MOUNTPOINT\t\tauto\tnoauto,owner,kudzu 0 0" \ >> /etc/fstab fi done if [ ! −z "$REMOVER" ]; then ## # Make sure this script is triggered on device removal. # mkdir −p `dirname $REMOVER` ln −s $IAM $REMOVER fi elif [ "${ACTION}" = "remove" ]; then ## # If the device is mounted, unmount it cleanly.
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Advanced Bash−Scripting Guide # if grep −q "$MOUNTPOINT" /etc/mtab; then # unmount cleanly umount −l $MOUNTPOINT fi ## # Remove it from /etc/fstab if it's there. # if grep −q "^$SYMLINKDEV" /etc/fstab; then grep −v "^$SYMLINKDEV" /etc/fstab > /etc/.fstab.new mv −f /etc/.fstab.new /etc/fstab fi fi exit 0
Here is something to warm the hearts of webmasters and mistresses everywhere: a script that saves weblogs.
Example A−23. Preserving weblogs #!/bin/bash # archiveweblogs.sh v1.0 # Troy Engel # Slightly modified by document author. # Used with permission. # # This script will preserve the normally rotated and #+ thrown away weblogs from a default RedHat/Apache installation. # It will save the files with a date/time stamp in the filename, #+ bzipped, to a given directory. # # Run this from crontab nightly at an off hour, #+ as bzip2 can suck up some serious CPU on huge logs: # 0 2 * * * /opt/sbin/archiveweblogs.sh
PROBLEM=66 # Set this to your backup dir. BKP_DIR=/opt/backups/weblogs # Default Apache/RedHat stuff LOG_DAYS="4 3 2 1" LOG_DIR=/var/log/httpd LOG_FILES="access_log error_log" # Default RedHat program locations LS=/bin/ls MV=/bin/mv ID=/usr/bin/id CUT=/bin/cut COL=/usr/bin/column BZ2=/usr/bin/bzip2 # Are we root? USER=`$ID −u` if [ "X$USER" != "X0" ]; then echo "PANIC: Only root can run this script!"
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451
Advanced Bash−Scripting Guide exit $PROBLEM fi # Backup dir exists/writable? if [ ! −x $BKP_DIR ]; then echo "PANIC: $BKP_DIR doesn't exist or isn't writable!" exit $PROBLEM fi # Move, rename and bzip2 the logs for logday in $LOG_DAYS; do for logfile in $LOG_FILES; do MYFILE="$LOG_DIR/$logfile.$logday" if [ −w $MYFILE ]; then DTS=`$LS −lgo −−time−style=+%Y%m%d $MYFILE | $COL −t | $CUT −d ' ' −f7` $MV $MYFILE $BKP_DIR/$logfile.$DTS $BZ2 $BKP_DIR/$logfile.$DTS else # Only spew an error if the file exits (ergo non−writable). if [ −f $MYFILE ]; then echo "ERROR: $MYFILE not writable. Skipping." fi fi done done exit 0
How do you keep the shell from expanding and reinterpreting strings?
Example A−24. Protecting literal strings #! /bin/bash # protect_literal.sh # set −vx :&1) #+ contain ONLY digits. # Parameters and Bash array elements are numbered. # (Parameters are very similar to Bash arrays.) # A variable name may be undefined (null reference). unset VarNull # A variable name may be defined but empty (null contents). VarEmpty='' # Two, adjacent, single quotes.
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Advanced Bash−Scripting Guide # A variable name my be defined and non−empty VarSomething='Literal' # A variable may contain: # * A whole number as a signed 32−bit (or larger) integer # * A string # A variable may also be an array. # A string may contain embedded blanks and may be treated #+ as if it where a function name with optional arguments. # The names of variables and the names of functions #+ are in different namespaces.
# A variable may be defined as a Bash array either explicitly or #+ implicitly by the syntax of the assignment statement. # Explicit: declare −a ArrayVar
# The echo command is a built−in. echo $VarSomething # The printf command is a built−in. # Translate %s as: String−Format printf %s $VarSomething # No linebreak specified, none output. echo # Default, only linebreak output.
# The Bash parser word breaks on whitespace. # Whitespace, or the lack of it is significant. # (This holds true in general; there are, of course, exceptions.)
# Translate the DOLLAR_SIGN character as: Content−Of. # Extended−Syntax way of writing Content−Of: echo ${VarSomething} # The ${ ... } Extended−Syntax allows more than just the variable #+ name to be specified. # In general, $VarSomething can always be written as: ${VarSomething}. # Call this script with arguments to see the following in action.
# Outside of double−quotes, the special characters @ and * #+ specify identical behavior. # May be pronounced as: All−Elements−Of. # Without specification of a name, they refer to the #+ pre−defined parameter Bash−Array.
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Advanced Bash−Scripting Guide # Glob−Pattern references echo $* echo ${*}
# All parameters to script or function # Same
# Bash disables filename expansion for Glob−Patterns. # Only character matching is active.
# All−Elements−Of references echo $@ echo ${@}
# Same as above # Same as above
# Within double−quotes, the behavior of Glob−Pattern references #+ depends on the setting of IFS (Input Field Separator). # Within double−quotes, All−Elements−Of references behave the same.
# Specifying only the name of a variable holding a string refers #+ to all elements (characters) of a string.
# To specify an element (character) of a string, #+ the Extended−Syntax reference notation (see below) MAY be used.
# Specifying only the name of a Bash array references #+ the subscript zero element, #+ NOT the FIRST DEFINED nor the FIRST WITH CONTENTS element. # Additional qualification is needed to reference other elements, #+ which means that the reference MUST be written in Extended−Syntax. # The general form is: ${name[subscript]}. # The string forms may also be used: ${name:subscript} #+ for Bash−Arrays when referencing the subscript zero element.
# Bash−Arrays are implemented internally as linked lists, #+ not as a fixed area of storage as in some programming languages.
# #
Characteristics of Bash arrays (Bash−Arrays): −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# #+ # ### # #+ ### # ### # ### # #
If not otherwise specified, Bash−Array subscripts begin with subscript number zero. Literally: [0] This is called zero−based indexing. If not otherwise specified, Bash−Arrays are subscript packed (sequential subscripts without subscript gaps). Negative subscripts are not allowed. Elements of a Bash−Array need not all be of the same type. Elements of a Bash−Array may be undefined (null reference). That is, a Bash−Array my be "subscript sparse."
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Advanced Bash−Scripting Guide ### # Elements of a Bash−Array may be defined and empty (null contents). ### # Elements of a Bash−Array may contain: # * A whole number as a signed 32−bit (or larger) integer # * A string # * A string formated so that it appears to be a function name # + with optional arguments ### # Defined elements of a Bash−Array may be undefined (unset). # That is, a subscript packed Bash−Array may be changed # + into a subscript sparse Bash−Array. ### # Elements may be added to a Bash−Array by defining an element #+ not previously defined. ### # For these reasons, I have been calling them "Bash−Arrays". # I'll return to the generic term "array" from now on. # −− msz
# Demo time −− initialize the previously declared ArrayVar as a #+ sparse array. # (The 'unset ... ' is just documentation here.) unset ArrayVar[0] ArrayVar[1]=one ArrayVar[2]='' unset ArrayVar[3] ArrayVar[4]='four'
# # # # #
Just for the record Unquoted literal Defined, and empty Just for the record Quoted literal
# Translate the %q format as: Quoted−Respecting−IFS−Rules. echo echo '− − Outside of double−quotes − −' ### printf %q ${ArrayVar[*]} # Glob−Pattern All−Elements−Of echo echo 'echo command:'${ArrayVar[*]} ### printf %q ${ArrayVar[@]} # All−Elements−Of echo echo 'echo command:'${ArrayVar[@]} # The use of double−quotes may be translated as: Enable−Substitution. # There are five cases recognized for the IFS setting. echo echo '− − Within double−quotes − Default IFS of space−tab−newline − −' IFS=$'\x20'$'\x09'$'\x0A' # These three bytes, #+ in exactly this order.
printf %q "${ArrayVar[*]}" # Glob−Pattern All−Elements−Of echo echo 'echo command:'"${ArrayVar[*]}" ### printf %q "${ArrayVar[@]}" # All−Elements−Of
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Advanced Bash−Scripting Guide echo echo 'echo command:'"${ArrayVar[@]}"
echo echo '− − Within double−quotes − First character of IFS is ^ − −' # Any printing, non−whitespace character should do the same. IFS='^'$IFS # ^ + space tab newline ### printf %q "${ArrayVar[*]}" # Glob−Pattern All−Elements−Of echo echo 'echo command:'"${ArrayVar[*]}" ### printf %q "${ArrayVar[@]}" # All−Elements−Of echo echo 'echo command:'"${ArrayVar[@]}"
echo echo '− − Within double−quotes − Without whitespace in IFS − −' IFS='^:%!' ### printf %q "${ArrayVar[*]}" # Glob−Pattern All−Elements−Of echo echo 'echo command:'"${ArrayVar[*]}" ### printf %q "${ArrayVar[@]}" # All−Elements−Of echo echo 'echo command:'"${ArrayVar[@]}"
echo echo '− − Within double−quotes − IFS set and empty − −' IFS='' ### printf %q "${ArrayVar[*]}" # Glob−Pattern All−Elements−Of echo echo 'echo command:'"${ArrayVar[*]}" ### printf %q "${ArrayVar[@]}" # All−Elements−Of echo echo 'echo command:'"${ArrayVar[@]}"
echo echo '− − Within double−quotes − IFS undefined − −' unset IFS ### printf %q "${ArrayVar[*]}" # Glob−Pattern All−Elements−Of echo echo 'echo command:'"${ArrayVar[*]}" ### printf %q "${ArrayVar[@]}" # All−Elements−Of echo echo 'echo command:'"${ArrayVar[@]}"
# Put IFS back to the default. # Default is exactly these three bytes. IFS=$'\x20'$'\x09'$'\x0A' # In exactly this order. # Interpretation of the above outputs:
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Advanced Bash−Scripting Guide # ### # ### # #+
A Glob−Pattern is I/O; the setting of IFS matters. An All−Elements−Of does not consider IFS settings. Note the different output using the echo command and the quoted format operator of the printf command.
# Recall: # Parameters are similar to arrays and have the similar behaviors. ### # The above examples demonstrate the possible variations. # To retain the shape of a sparse array, additional script #+ programming is required. ### # The source code of Bash has a routine to output the #+ [subscript]=value array assignment format. # As of version 2.05b, that routine is not used, #+ but that might change in future releases.
# The length of a string, measured in non−null elements (characters): echo echo '− − Non−quoted references − −' echo 'Non−Null character count: '${#VarSomething}' characters.' # test='Lit'$'\x00''eral' # echo ${#test}
# $'\x00' is a null character. # See that?
# The length of an array, measured in defined elements, #+ including null content elements. echo echo 'Defined content count: '${#ArrayVar[@]}' elements.' # That is NOT the maximum subscript (4). # That is NOT the range of the subscripts (1 . . 4 inclusive). # It IS the length of the linked list. ### # Both the maximum subscript and the range of the subscripts may #+ be found with additional script programming. # The length of a string, measured in non−null elements (characters): echo echo '− − Quoted, Glob−Pattern references − −' echo 'Non−Null character count: '"${#VarSomething}"' characters.' # The length of an array, measured in defined elements, #+ including null−content elements. echo echo 'Defined element count: '"${#ArrayVar[*]}"' elements.' # # # #+
Interpretation: Substitution does not effect the ${# ... } operation. Suggestion: Always use the All−Elements−Of character if that is what is intended (independence from IFS).
# #
Define a simple function. I include an underscore in the name
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Advanced Bash−Scripting Guide #+ to make it distinctive in the examples below. ### # Bash separates variable names and function names #+ in different namespaces. # The Mark−One eyeball isn't that advanced. ### _simple() { echo −n 'SimpleFunc'$@ # Newlines are swallowed in } #+ result returned in any case.
# The ( ... ) notation invokes a command or function. # The $( ... ) notation is pronounced: Result−Of.
# Invoke the function _simple echo echo '− − Output of function _simple − −' _simple # Try passing arguments. echo # or (_simple) # Try passing arguments. echo echo '− Is there a variable of that name? −' echo $_simple not defined # No variable by that name. # Invoke the result of function _simple (Error msg intended) ### $(_simple) # #
# Gives an error message: line 394: SimpleFunc: command not found −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
echo ### # The first word of the result of function _simple #+ is neither a valid Bash command nor the name of a defined function. ### # This demonstrates that the output of _simple is subject to evaluation. ### # Interpretation: # A function can be used to generate in−line Bash commands.
# A simple function where the first word of result IS a bash command: ### _print() { echo −n 'printf %q '$@ } echo '− − Outputs of function _print − −' _print parm1 parm2 # An Output NOT A Command. echo $(_print parm1 parm2)
# Executes: printf %q parm1 parm2 # See above IFS examples for the #+ various possibilities.
echo $(_print $VarSomething)
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# The predictable result.
463
Advanced Bash−Scripting Guide echo
# Function variables # −−−−−−−−−−−−−−−−−− echo echo '− − Function variables − −' # A variable may represent a signed integer, a string or an array. # A string may be used like a function name with optional arguments. # set −vx declare −f funcVar
# Enable if desired #+ in namespace of functions
funcVar=_print $funcVar parm1 echo
# Contains name of function. # Same as _print at this point.
funcVar=$(_print ) $funcVar $funcVar $VarSomething echo
# Contains result of function. # No input, No output. # The predictable result.
funcVar=$(_print $VarSomething) $funcVar echo
# $VarSomething replaced HERE. # The expansion is part of the #+ variable contents.
funcVar="$(_print $VarSomething)" $funcVar echo
# $VarSomething replaced HERE. # The expansion is part of the #+ variable contents.
# #+ # #
The difference between the unquoted and the double−quoted versions above can be seen in the "protect_literal.sh" example. The first case above is processed as two, unquoted, Bash−Words. The second case above is processed as one, quoted, Bash−Word.
# Delayed replacement # −−−−−−−−−−−−−−−−−−− echo echo '− − Delayed replacement − −' funcVar="$(_print '$VarSomething')" # No replacement, single Bash−Word. eval $funcVar # $VarSomething replaced HERE. echo VarSomething='NewThing' eval $funcVar echo
# $VarSomething replaced HERE.
# Restore the original setting trashed above. VarSomething=Literal # #+ # #+
There are a pair of functions demonstrated in the "protect_literal.sh" and "unprotect_literal.sh" examples. These are general purpose functions for delayed replacement literals containing variables.
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Advanced Bash−Scripting Guide
# REVIEW: # −−−−−− # A string can be considered a Classic−Array of elements (characters). # A string operation applies to all elements (characters) of the string #+ (in concept, anyway). ### # The notation: ${array_name[@]} represents all elements of the #+ Bash−Array: array_name. ### # The Extended−Syntax string operations can be applied to all #+ elements of an array. ### # This may be thought of as a For−Each operation on a vector of strings. ### # Parameters are similar to an array. # The initialization of a parameter array for a script #+ and a parameter array for a function only differ #+ in the initialization of ${0}, which never changes its setting. ### # Subscript zero of the script's parameter array contains #+ the name of the script. ### # Subscript zero of a function's parameter array DOES NOT contain #+ the name of the function. # The name of the current function is accessed by the $FUNCNAME variable. ### # A quick, review list follows (quick, not short). echo echo echo echo echo echo echo
'− − Test (but not change) − −' '− null reference −' −n ${VarNull−'NotSet'}' ' ${VarNull} −n ${VarNull:−'NotSet'}' ' ${VarNull}
# # # #
NotSet NewLine only NotSet Newline only
echo echo echo echo echo
'− null contents −' −n ${VarEmpty−'Empty'}' ' ${VarEmpty} −n ${VarEmpty:−'Empty'}' ' ${VarEmpty}
# # # #
Only the space Newline only Empty Newline only
echo '− contents −' echo ${VarSomething−'Content'} echo ${VarSomething:−'Content'}
# Literal # Literal
echo '− Sparse Array −' echo ${ArrayVar[@]−'not set'} # # # # # #
ASCII−Art time State Y==yes, − Unset Y Empty N Contents N
N==no :− Y Y N
${# ... } == 0 ${# ... } == 0 ${# ... } > 0
# Either the first and/or the second part of the tests #+ may be a command or a function invocation string.
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Advanced Bash−Scripting Guide echo echo '− − Test 1 for undefined − −' declare −i t _decT() { t=$t−1 } # Null reference, set: t == −1 t=${#VarNull} ${VarNull− _decT } echo $t
# Results in zero. # Function executes, t now −1.
# Null contents, set: t == 0 t=${#VarEmpty} ${VarEmpty− _decT } echo $t
# Results in zero. # _decT function NOT executed.
# Contents, set: t == number of non−null characters VarSomething='_simple' # Set to valid function name. t=${#VarSomething} # non−zero length ${VarSomething− _decT } # Function _simple executed. echo $t # Note the Append−To action. # Exercise: clean up that example. unset t unset _decT VarSomething=Literal echo echo '− − Test and Change − −' echo '− Assignment if null reference −' echo −n ${VarNull='NotSet'}' ' # NotSet NotSet echo ${VarNull} unset VarNull echo '− Assignment if null reference −' echo −n ${VarNull:='NotSet'}' ' # NotSet NotSet echo ${VarNull} unset VarNull echo '− No assignment if null contents −' echo −n ${VarEmpty='Empty'}' ' # Space only echo ${VarEmpty} VarEmpty='' echo '− Assignment if null contents −' echo −n ${VarEmpty:='Empty'}' ' echo ${VarEmpty} VarEmpty=''
# Empty Empty
echo '− No change if already has contents −' echo ${VarSomething='Content'} # Literal echo ${VarSomething:='Content'} # Literal
# "Subscript sparse" Bash−Arrays ### # Bash−Arrays are subscript packed, beginning with #+ subscript zero unless otherwise specified. ### # The initialization of ArrayVar was one way #+ to "otherwise specify". Here is the other way:
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Advanced Bash−Scripting Guide ### echo declare −a ArraySparse ArraySparse=( [1]=one [2]='' [4]='four' ) # [0]=null reference, [2]=null content, [3]=null reference echo '− − Array−Sparse List − −' # Within double−quotes, default IFS, Glob−Pattern IFS=$'\x20'$'\x09'$'\x0A' printf %q "${ArraySparse[*]}" echo # Note that the output does not distinguish between "null content" #+ and "null reference". # Both print as escaped whitespace. ### # Note also that the output does NOT contain escaped whitespace #+ for the "null reference(s)" prior to the first defined element. ### # This behavior of 2.04, 2.05a and 2.05b has been reported #+ and may change in a future version of Bash. # To output a sparse array and maintain the [subscript]=value #+ relationship without change requires a bit of programming. # One possible code fragment: ### # local l=${#ArraySparse[@]} # Count of defined elements # local f=0 # Count of found subscripts # local i=0 # Subscript to test ( # Anonymous in−line function for (( l=${#ArraySparse[@]}, f = 0, i = 0 ; f < l ; i++ )) do # 'if defined then...' ${ArraySparse[$i]+ eval echo '\ ['$i']='${ArraySparse[$i]} ; (( f++ )) } done ) # The reader coming upon the above code fragment cold #+ might want to review "command lists" and "multiple commands on a line" #+ in the text of the foregoing "Advanced Bash Scripting Guide." ### # Note: # The "read −a array_name" version of the "read" command #+ begins filling array_name at subscript zero. # ArraySparse does not define a value at subscript zero. ### # The user needing to read/write a sparse array to either #+ external storage or a communications socket must invent #+ a read/write code pair suitable for their purpose. ### # Exercise: clean it up. unset ArraySparse echo echo '− − Conditional alternate (But not change)− −' echo '− No alternate if null reference −' echo −n ${VarNull+'NotSet'}' ' echo ${VarNull} unset VarNull
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Advanced Bash−Scripting Guide echo '− No alternate if null reference −' echo −n ${VarNull:+'NotSet'}' ' echo ${VarNull} unset VarNull echo '− Alternate if null contents −' echo −n ${VarEmpty+'Empty'}' ' echo ${VarEmpty} VarEmpty='' echo '− No alternate if null contents −' echo −n ${VarEmpty:+'Empty'}' ' echo ${VarEmpty} VarEmpty=''
# Empty
# Space only
echo '− Alternate if already has contents −' # Alternate literal echo −n ${VarSomething+'Content'}' ' echo ${VarSomething}
# Content Literal
# Invoke function echo −n ${VarSomething:+ $(_simple) }' ' echo ${VarSomething} echo echo '− − Sparse Array − −' echo ${ArrayVar[@]+'Empty'} echo
# SimpleFunc Literal
# An array of 'Empty'(ies)
echo '− − Test 2 for undefined − −' declare −i t _incT() { t=$t+1 } # Note: # This is the same test used in the sparse array #+ listing code fragment. # Null reference, set: t == −1 t=${#VarNull}−1 ${VarNull+ _incT } echo $t' Null reference'
# Results in minus−one. # Does not execute.
# Null contents, set: t == 0 t=${#VarEmpty}−1 ${VarEmpty+ _incT } echo $t' Null content'
# Results in minus−one. # Executes.
# Contents, set: t == (number of non−null characters) t=${#VarSomething}−1 # non−null length minus−one ${VarSomething+ _incT } # Executes. echo $t' Contents' # Exercise: clean up that example. unset t unset _incT # ${name?err_msg} ${name:?err_msg} # These follow the same rules but always exit afterwards
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Advanced Bash−Scripting Guide #+ if an action is specified following the question mark. # The action following the question mark may be a literal #+ or a function result. ### # ${name?} ${name:?} are test−only, the return can be tested.
# Element operations # −−−−−−−−−−−−−−−−−− echo echo '− − Trailing sub−element selection − −' #
Strings, Arrays and Positional parameters
# Call this script with multiple arguments #+ to see the parameter selections. echo echo echo echo
'− All −' ${VarSomething:0} ${ArrayVar[@]:0} ${@:0}
echo echo echo echo echo
'− All after −' ${VarSomething:1} ${ArrayVar[@]:1} ${@:2}
echo echo '− Range after −' echo ${VarSomething:4:3}
# # # #
all non−null characters all elements with content all parameters with content; ignoring parameter[0]
# all non−null after character[0] # all after element[0] with content # all after param[1] with content
# ral # Three characters after # character[3]
echo '− Sparse array gotch −' echo ${ArrayVar[@]:1:2} # four − The only element with content. # Two elements after (if that many exist). # the FIRST WITH CONTENTS #+ (the FIRST WITH CONTENTS is being #+ considered as if it #+ were subscript zero). # Executed as if Bash considers ONLY array elements with CONTENT # printf %q "${ArrayVar[@]:0:3}" # Try this one # #+ # # #+
In versions 2.04, 2.05a and 2.05b, Bash does not handle sparse arrays as expected using this notation. The current Bash maintainer, Chet Ramey, has corrected this for an upcoming version of Bash.
echo '− Non−sparse array −' echo ${@:2:2} # Two parameters following parameter[1] # New victims for string vector examples: stringZ=abcABC123ABCabc arrayZ=( abcabc ABCABC 123123 ABCABC abcabc ) sparseZ=( [1]='abcabc' [3]='ABCABC' [4]='' [5]='123123' )
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Advanced Bash−Scripting Guide echo echo echo echo echo echo echo
' ' ' ' ' '
− − − − − −
− Victim string − −'$stringZ'− − ' − Victim array − −'${arrayZ[@]}'− − ' − Sparse array − −'${sparseZ[@]}'− − ' [0]==null ref, [2]==null ref, [4]==null content − ' [1]=abcabc [3]=ABCABC [5]=123123 − ' non−null−reference count: '${#sparseZ[@]}' elements'
echo echo '− − Prefix sub−element removal − −' echo '− − Glob−Pattern match must include the first character. − −' echo '− − Glob−Pattern may be a literal or a function result. − −' echo
# Function returning a simple, Literal, Glob−Pattern _abc() { echo −n 'abc' } echo echo echo echo
'− Shortest prefix −' ${stringZ#123} ${stringZ#$(_abc)} ${arrayZ[@]#abc}
# Unchanged (not a prefix). # ABC123ABCabc # Applied to each element.
# Fixed by Chet Ramey for an upcoming version of Bash. # echo ${sparseZ[@]#abc} # Version−2.05b core dumps. # The −it would be nice− First−Subscript−Of # echo ${#sparseZ[@]#*} # This is NOT valid Bash. echo echo echo echo echo
'− Longest prefix −' ${stringZ##1*3} ${stringZ##a*C} ${arrayZ[@]##a*c}
# Unchanged (not a prefix) # abc # ABCABC 123123 ABCABC
# Fixed by Chet Ramey for an upcoming version of Bash # echo ${sparseZ[@]##a*c} # Version−2.05b core dumps. echo echo echo echo echo echo echo echo echo
'− − Suffix sub−element removal − −' '− − Glob−Pattern match must include the last character. − −' '− − Glob−Pattern may be a literal or a function result. − −' '− Shortest suffix −' ${stringZ%1*3} ${stringZ%$(_abc)} ${arrayZ[@]%abc}
# Unchanged (not a suffix). # abcABC123ABC # Applied to each element.
# Fixed by Chet Ramey for an upcoming version of Bash. # echo ${sparseZ[@]%abc} # Version−2.05b core dumps. # The −it would be nice− Last−Subscript−Of # echo ${#sparseZ[@]%*} # This is NOT valid Bash. echo echo echo echo echo
'− Longest suffix −' ${stringZ%%1*3} ${stringZ%%b*c} ${arrayZ[@]%%b*c}
Appendix A. Contributed Scripts
# Unchanged (not a suffix) # a # a ABCABC 123123 ABCABC a
470
Advanced Bash−Scripting Guide # Fixed by Chet Ramey for an upcoming version of Bash. # echo ${sparseZ[@]%%b*c} # Version−2.05b core dumps. echo echo echo echo echo echo echo echo echo
'− '− '− '− '− '− '
− − − − − −
Sub−element replacement − −' Sub−element at any location in string. − −' First specification is a Glob−Pattern − −' Glob−Pattern may be a literal or Glob−Pattern function result. − −' Second specification may be a literal or function result. − −' Second specification may be unspecified. Pronounce that' as: Replace−With−Nothing (Delete) − −'
# Function returning a simple, Literal, Glob−Pattern _123() { echo −n '123' } echo echo echo echo echo
'− Replace first occurrence −' ${stringZ/$(_123)/999} # Changed (123 is a component). ${stringZ/ABC/xyz} # xyzABC123ABCabc ${arrayZ[@]/ABC/xyz} # Applied to each element. ${sparseZ[@]/ABC/xyz} # Works as expected.
echo echo echo echo echo echo
'− Delete first occurrence −' ${stringZ/$(_123)/} ${stringZ/ABC/} ${arrayZ[@]/ABC/} ${sparseZ[@]/ABC/}
# The replacement need not be a literal, #+ since the result of a function invocation is allowed. # This is general to all forms of replacement. echo echo '− Replace first occurrence with Result−Of −' echo ${stringZ/$(_123)/$(_simple)} # Works as expected. echo ${arrayZ[@]/ca/$(_simple)} # Applied to each element. echo ${sparseZ[@]/ca/$(_simple)} # Works as expected. echo echo echo echo echo echo
'− Replace all occurrences −' ${stringZ//[b2]/X} ${stringZ//abc/xyz} ${arrayZ[@]//abc/xyz} ${sparseZ[@]//abc/xyz}
echo echo echo echo echo echo
'− Delete all occurrences −' ${stringZ//[b2]/} ${stringZ//abc/} ${arrayZ[@]//abc/} ${sparseZ[@]//abc/}
# # # #
X−out b's and 2's xyzABC123ABCxyz Applied to each element. Works as expected.
echo echo '− − Prefix sub−element replacement − −' echo '− − Match must include the first character. − −' echo
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Advanced Bash−Scripting Guide echo echo echo echo echo
'− Replace prefix occurrences −' ${stringZ/#[b2]/X} # Unchanged (neither is a prefix). ${stringZ/#$(_abc)/XYZ} # XYZABC123ABCabc ${arrayZ[@]/#abc/XYZ} # Applied to each element. ${sparseZ[@]/#abc/XYZ} # Works as expected.
echo echo echo echo echo echo
'− Delete prefix occurrences −' ${stringZ/#[b2]/} ${stringZ/#$(_abc)/} ${arrayZ[@]/#abc/} ${sparseZ[@]/#abc/}
echo echo '− − Suffix sub−element replacement − −' echo '− − Match must include the last character. − −' echo echo echo echo echo echo
'− Replace suffix occurrences −' ${stringZ/%[b2]/X} # Unchanged (neither is a suffix). ${stringZ/%$(_abc)/XYZ} # abcABC123ABCXYZ ${arrayZ[@]/%abc/XYZ} # Applied to each element. ${sparseZ[@]/%abc/XYZ} # Works as expected.
echo echo echo echo echo echo
'− Delete suffix occurrences −' ${stringZ/%[b2]/} ${stringZ/%$(_abc)/} ${arrayZ[@]/%abc/} ${sparseZ[@]/%abc/}
echo echo '− − Special cases of null Glob−Pattern − −' echo echo '− Prefix all −' # null substring pattern means 'prefix' echo ${stringZ/#/NEW} # NEWabcABC123ABCabc echo ${arrayZ[@]/#/NEW} # Applied to each element. echo ${sparseZ[@]/#/NEW} # Applied to null−content also. # That seems reasonable. echo echo '− Suffix all −' # null substring pattern means 'suffix' echo ${stringZ/%/NEW} # abcABC123ABCabcNEW echo ${arrayZ[@]/%/NEW} # Applied to each element. echo ${sparseZ[@]/%/NEW} # Applied to null−content also. # That seems reasonable. echo echo '− − Special case For−Each Glob−Pattern − −' echo '− − − − This is a nice−to−have dream − − − −' echo _GenFunc() { echo −n ${0} # Illustration only. # Actually, that would be an arbitrary computation. } # All occurrences, matching the AnyThing pattern. # Currently //*/ does not match null−content nor null−reference.
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Advanced Bash−Scripting Guide # /#/ and /%/ does match null−content but not null−reference. echo ${sparseZ[@]//*/$(_GenFunc)}
# A possible syntax would be to make #+ the parameter notation used within this construct mean: # ${1} − The full element # ${2} − The prefix, if any, to the matched sub−element # ${3} − The matched sub−element # ${4} − The suffix, if any, to the matched sub−element # # echo ${sparseZ[@]//*/$(_GenFunc ${3})} # Same as ${1} here. # Perhaps it will be implemented in a future version of Bash.
exit 0
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Appendix B. Reference Cards The following reference cards provide a useful summary of certain scripting concepts. The foregoing text treats these matters in more depth and gives usage examples.
Table B−1. Special Shell Variables Variable $0 $1 $2 − $9 ${10} $# "$*" "$@" ${#*} ${#@} $? $$ $− $_ $!
Meaning Name of script Positional parameter #1 Positional parameters #2 − #9 Positional parameter #10 Number of positional parameters All the positional parameters (as a single word) * All the positional parameters (as separate strings) Number of command line parameters passed to script Number of command line parameters passed to script Return value Process ID (PID) of script Flags passed to script (using set) Last argument of previous command Process ID (PID) of last job run in background
* Must be quoted, otherwise it defaults to "$@".
Table B−2. TEST Operators: Binary Comparison Operator
Meaning
Arithmetic Comparison −eq Equal to −ne −lt −le −gt −ge
Not equal to Less than Less than or equal to Greater than Greater than or equal to
−−−−− Operator
Meaning
String Comparison = == != \
Greater than (ASCII) *
−z −n
String is empty String is not empty
Arithmetic Comparison within double parentheses (( ... ))
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474
Advanced Bash−Scripting Guide > >= < (COMMAND) Process substitution &1 # For example: ./letter−count.sh filename.txt a b c exit $E_PARAMERR # Not enough arguments passed to script. } if [ ! −f "$1" ] ; then echo "$1: No such file." 2>&1 usage # Print usage message and exit. fi if [ −z "$2" ] ; then echo "$2: No letters specified." 2>&1 usage fi
shift # Letters specified. for letter in `echo $@` # For each one . . . do INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \"$letter\"; final_tab[${count_case}] = # Pass as parameter to awk script below. count_case=`expr $count_case + 1` done # DEBUG: # echo $INIT_TAB_AWK; cat $FILE_PARSE | # Pipe the target file to the following awk script. # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− awk −v tab_search=0 −v final_tab=0 −v tab=0 −v nb_letter=0 −v chara=0 −v chara2=0 \ "BEGIN { $INIT_TAB_AWK } \ { split(\$0, tab, \"\"); \ for (chara in tab) \ { for (chara2 in tab_search) \ { if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \ END { for (chara in final_tab) \ { print tab_search[chara] \" => \" final_tab[chara] } }" # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Nothing all that complicated, just . . . #+ for−loops, if−tests, and a couple of specialized functions. exit $?
For simpler examples of awk within shell scripts, see: 1. Example 11−11 2. Example 16−7 3. Example 12−27 4. Example 34−3 5. Example 9−22 6. Example 11−17 7. Example 28−2 8. Example 28−3 9. Example 10−3 10. Example 12−48 Appendix C. A Sed and Awk Micro−Primer
483
Advanced Bash−Scripting Guide 11. Example 9−27 12. Example 12−4 13. Example 9−12 14. Example 34−12 15. Example 10−8 That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the Bibliography.
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Appendix D. Exit Codes With Special Meanings Table D−1. "Reserved" Exit Codes Exit Code Number 1 2 126 127 128 128+n 130 255*
Meaning
Example
Comments
catchall for general errors
let "var1 = 1/0" miscellaneous errors, such as "divide by zero" misuse of shell builtins, according to Seldom seen, usually defaults to exit Bash documentation code 1 command invoked cannot execute permission problem or command is not an executable "command not found" possible problem with $PATH or a typo invalid argument to exit exit 3.14159 exit takes only integer args in the range 0 − 255 fatal error signal "n" kill −9 $PPID $? returns 137 (128 + 9) of script script terminated by Control−C Control−C is fatal error signal 2, (130 = 128 + 2, see above) exit status out of range exit −1 exit takes only integer args in the range 0 − 255
According to the table, exit codes 1 − 2, 126 − 165, and 255 [72] have special meanings, and should therefore be avoided as user−specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error a "command not found" or a user−defined one?). However, many scripts use an exit 1 as a general bailout upon error. Since exit code 1 signifies so many possible errors, this might not add any additional ambiguity, but, on the other hand, it probably would not be very informative either. There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of this document proposes restricting user−defined exit codes to the range 64 − 113 (in addition to 0, for success), to conform with the C/C++ standard. This would allot 50 valid codes, and make troubleshooting scripts more straightforward. All user−defined exit codes in the accompanying examples to this document now conform to this standard, except where overriding circumstances exist, as in Example 9−2. Issuing a $? from the command line after a shell script exits gives results consistent with the table above only from the Bash or sh prompt. Running the C−shell or tcsh may give different values in some cases.
Appendix D. Exit Codes With Special Meanings
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Appendix E. A Detailed Introduction to I/O and I/O Redirection written by Stephane Chazelas, and revised by the document author A command expects the first three file descriptors to be available. The first, fd 0 (standard input, stdin), is for reading. The other two (fd 1, stdout and fd 2, stderr) are for writing. There is a stdin, stdout, and a stderr associated with each command. ls 2>&1 means temporarily connecting the stderr of the ls command to the same "resource" as the shell's stdout. By convention, a command reads its input from fd 0 (stdin), prints normal output to fd 1 (stdout), and error ouput to fd 2 (stderr). If one of those three fd's is not open, you may encounter problems: bash$ cat /etc/passwd >&− cat: standard output: Bad file descriptor
For example, when xterm runs, it first initializes itself. Before running the user's shell, xterm opens the terminal device (/dev/pts/ or something similar) three times. At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits them in turn, except when you redirect the command. Redirection means reassigning one of the file descriptors to another file (or a pipe, or anything permissible). File descriptors may be reassigned locally (for a command, a command group, a subshell, a while or if or case or for loop...), or globally, for the remainder of the shell (using exec). ls > /dev/null means running ls with its fd 1 connected to /dev/null. bash$ lsof −a −p $$ −d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 363 bozo 0u CHR 136,1 3 /dev/pts/1 bash 363 bozo 1u CHR 136,1 3 /dev/pts/1 bash 363 bozo 2u CHR 136,1 3 /dev/pts/1
bash$ exec 2> /dev/null bash$ lsof −a −p $$ −d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 371 bozo 0u CHR 136,1 3 /dev/pts/1 bash 371 bozo 1u CHR 136,1 3 /dev/pts/1 bash 371 bozo 2w CHR 1,3 120 /dev/null
bash$ bash −c 'lsof −a −p $$ −d0,1,2' | cat COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 379 root 0u CHR 136,1 3 /dev/pts/1 lsof 379 root 1w FIFO 0,0 7118 pipe lsof 379 root 2u CHR 136,1 3 /dev/pts/1
bash$ echo "$(bash −c 'lsof −a −p $$ −d0,1,2' 2>&1)" COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 426 root 0u CHR 136,1 3 /dev/pts/1
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Advanced Bash−Scripting Guide lsof lsof
426 root 426 root
1w 2w
FIFO FIFO
0,0 0,0
7520 pipe 7520 pipe
This works for different types of redirection. Exercise: Analyze the following script. #! /usr/bin/env bash mkfifo /tmp/fifo1 /tmp/fifo2 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)
exec 3>&1 ( ( ( while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr | tee /dev/fd/4 | tee / exec 3> /tmp/fifo2 echo 1st, sleep 1 echo 2nd, sleep 1 echo 3rd, sleep 1 echo 4th, sleep 1 echo 5th, sleep 1 echo 6th, sleep 1 echo 7th, sleep 1 echo 8th, sleep 1 echo 9th,
to stdout to stderr >&2 to fd 3 >&3 to fd 4 >&4 to fd 5 >&5 through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5 to fd 6 >&6 to fd 7 >&7 to fd 8 >&8
) 4>&1 >&3 3>&− | while read a; do echo "FD4: $a"; done 1>&3 5>&− 6>&− ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&− ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&− rm −f /tmp/fifo1 /tmp/fifo2
# For each command and subshell, figure out which fd points to what. exit 0
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Appendix F. Standard Command−Line Options Over time, there has evolved a loose standard for the meanings of command line option flags. The GNU utilities conform more closely to this "standard" than older Unix utilities. Traditionally, Unix command−line options consist of a dash, followed by one or more lowercase letters. The GNU utilities added a double−dash, followed by a complete word or compound word. The two most widely−accepted options are: • −h −−help Help: Give usage message and exit. • −v −−version Version: Show program version and exit. Other common options are: • −a −−all All: show all information or operate on all arguments. • −l −−list List: list files or arguments without taking other action. • −o Output filename • −q −−quiet Quiet: suppress stdout. • −r −R −−recursive Recursive: Operate recursively (down directory tree). • −v
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Advanced Bash−Scripting Guide −−verbose Verbose: output additional information to stdout or stderr. • −z −−compress Compress: apply compression (usually gzip). However: • In tar and gawk: −f −−file File: filename follows. • In cp, mv, rm: −f −−force Force: force overwrite of target file(s). Many Unix and Linux utilities deviate from this "standard," so it is dangerous to assume that a given option will behave in a standard way. Always check the man page for the command in question when in doubt. A complete table of recommended options for the GNU utilities is available at http://www.gnu.org/prep/standards_19.html.
Appendix F. Standard Command−Line Options
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Appendix G. Important System Directories Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories. • /bin Binary executables. Basic system programs and utilities (such as bash). • /usr/bin [73] More system executables. • /usr/local/bin Miscellaneous executables. • /sbin Superuser binaries. Basic system administrative programs and utilities (such as fsck). • /usr/sbin More superuser binaries. • /etc Et cetera. Systemwide configuration scripts. • /etc/rc.d Boot scripts, on Red Hat and derivative distributions of Linux. • /usr/share/doc Documentation for installed packages. • /tmp System temporary files. • /var/log Systemwide log files. • /var/spool/mail User mail spool.
Appendix G. Important System Directories
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Appendix H. Localization Localization is an undocumented Bash feature. A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get output from the same script in English. To create a localized script, use the following template to write all messages to the user (error messages, prompts, etc.). #!/bin/bash # localized.sh # Script by Stephane Chazelas, modified by Bruno Haible . gettext.sh E_CDERROR=65 error() { printf "$@" >&2 exit $E_CDERROR } cd $var || error "`eval_gettext \"Can't cd to \$var.\"`" read −p "`gettext \"Enter the value: \"`" var # ... bash$ bash −D localized.sh "Can't cd to %s." "Enter the value: "
This lists all the localized text. (The −D option lists double−quoted strings prefixed by a $, without executing the script.) bash$ bash −−dump−po−strings localized.sh #: a:6 msgid "Can't cd to %s." msgstr "" #: a:7 msgid "Enter the value: " msgstr ""
The −−dump−po−strings option to Bash resembles the −D option, but uses gettext "po" format. Bruno Haible points out: Starting with gettext−0.12.2, xgettext −o − localized.sh is recommended instead of bash −−dump−po−strings localized.sh, because xgettext . . . 1. understands the gettext and eval_gettext commands (whereas bash −−dump−po−strings understands only its deprecated $"..." syntax)
Appendix H. Localization
491
Advanced Bash−Scripting Guide 2. can extract comments placed by the programmer, intended to be read by the translator. This shell code is then not specific to Bash any more; it works the same way with Bash 1.x and other /bin/sh implementations. Now, build a language.po file for each language that the script will be translated into, specifying the msgstr. As an example: fr.po: #: a:6 msgid "Can't cd to %s." msgstr "Impossible de se positionner dans le répertoire %s." #: a:7 msgid "Enter the value: " msgstr "Entrez la valeur : "
Then, run msgfmt. msgfmt −o localized.sh.mo fr.po Place the resulting localized.sh.mo file in the /usr/local/share/locale/fr/LC_MESSAGES directory, and at the beginning of the script, insert the lines: TEXTDOMAINDIR=/usr/local/share/locale TEXTDOMAIN=localized.sh
If a user on a French system runs the script, she will get French messages. With older versions of Bash or other shells, localization requires gettext, using the −s option. In this case, the script becomes:
#!/bin/bash # localized.sh E_CDERROR=65 error() { local format=$1 shift printf "$(gettext −s "$format")" "$@" >&2 exit $E_CDERROR } cd $var || error "Can't cd to %s." "$var" read −p "$(gettext −s "Enter the value: ")" var # ...
The TEXTDOMAIN and TEXTDOMAINDIR variables need to be exported to the environment. −−− This appendix written by Stephane Chazelas, with modifications suggested by Bruno Haible, maintainer of GNU gettext. Appendix H. Localization
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Appendix H. Localization
493
Appendix I. History Commands The Bash shell provides command−line tools for editing and manipulating a user's command history. This is primarily a convenience, a means of saving keystrokes. Bash history commands: 1. history 2. fc bash$ history 1 mount /mnt/cdrom 2 cd /mnt/cdrom 3 ls ...
Internal variables associated with Bash history commands: 1. $HISTCMD 2. $HISTCONTROL 3. $HISTIGNORE 4. $HISTFILE 5. $HISTFILESIZE 6. $HISTSIZE 7. !! 8. !$ 9. !# 10. !N 11. !−N 12. !STRING 13. !?STRING? 14. ^STRING^string^ Unfortunately, the Bash history tools find no use in scripting. #!/bin/bash # history.sh # Attempt to use 'history' command in a script. history # Script produces no output. # History commands do not work within a script. bash$ ./history.sh (no output)
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Appendix J. A Sample .bashrc File The ~/.bashrc file determines the behavior of interactive shells. A good look at this file can lead to a better understanding of Bash. Emmanuel Rouat contributed the following very elaborate .bashrc file, written for a Linux system. He welcomes reader feedback on it. Study the file carefully, and feel free to reuse code snippets and functions from it in your own .bashrc file or even in your scripts.
Example J−1. Sample .bashrc file #=============================================================== # # PERSONAL $HOME/.bashrc FILE for bash−2.05a (or later) # # Last modified: Tue Apr 15 20:32:34 CEST 2003 # # This file is read (normally) by interactive shells only. # Here is the place to define your aliases, functions and # other interactive features like your prompt. # # This file was designed (originally) for Solaris but based # on Redhat's default .bashrc file # −−> Modified for Linux. # The majority of the code you'll find here is based on code found # on Usenet (or internet). # This bashrc file is a bit overcrowded − remember it is just # just an example. Tailor it to your needs # # #=============================================================== # −−> Comments added by HOWTO author. # −−> And then edited again by ER :−) #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Source global definitions (if any) #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− if [ −f /etc/bashrc ]; then . /etc/bashrc # −−> Read /etc/bashrc, if present. fi #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Automatic setting of $DISPLAY (if not set already) # This works for linux − your mileage may vary.... # The problem is that different types of terminals give # different answers to 'who am i'...... # I have not found a 'universal' method yet #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− function get_xserver () { case $TERM in xterm )
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Advanced Bash−Scripting Guide XSERVER=$(who am i | awk '{print $NF}' | tr −d ')''(' ) XSERVER=${XSERVER%%:*} ;; aterm | rxvt) # find some code that works here..... ;; esac } if [ −z ${DISPLAY:=""} ]; then get_xserver if [[ −z ${XSERVER} || ${XSERVER} == $(hostname) || ${XSERVER} == "unix" ]]; then DISPLAY=":0.0" # Display on local host else DISPLAY=${XSERVER}:0.0 # Display on remote host fi fi export DISPLAY #−−−−−−−−−−−−−−− # Some settings #−−−−−−−−−−−−−−− ulimit −S −c 0 set −o notify set −o noclobber set −o ignoreeof set −o nounset #set −o xtrace # Enable shopt −s shopt −s shopt −s shopt −s shopt −s shopt −s shopt −s shopt −s shopt −s shopt −s
# Don't want any coredumps
# useful for debuging
options: cdspell cdable_vars checkhash checkwinsize mailwarn sourcepath no_empty_cmd_completion # bash>=2.04 only cmdhist histappend histreedit histverify extglob # necessary for programmable completion
# Disable options: shopt −u mailwarn unset MAILCHECK
# I don't want my shell to warn me of incoming mail
export TIMEFORMAT=$'\nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n' export HISTIGNORE="&:bg:fg:ll:h" export HOSTFILE=$HOME/.hosts # Put a list of remote hosts in ~/.hosts
#−−−−−−−−−−−−−−−−−−−−−−− # Greeting, motd etc... #−−−−−−−−−−−−−−−−−−−−−−− # Define some colors first: red='\e[0;31m' RED='\e[1;31m' blue='\e[0;34m'
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Advanced Bash−Scripting Guide BLUE='\e[1;34m' cyan='\e[0;36m' CYAN='\e[1;36m' NC='\e[0m' # No Color # −−> Nice. Has the same effect as using "ansi.sys" in DOS. # Looks best on a black background..... echo −e "${CYAN}This is BASH ${RED}${BASH_VERSION%.*}${CYAN} − DISPLAY on ${RED}$DISPLAY${NC}\n" date if [ −x /usr/games/fortune ]; then /usr/games/fortune −s # makes our day a bit more fun.... :−) fi function _exit() # function to run upon exit of shell { echo −e "${RED}Hasta la vista, baby${NC}" } trap _exit EXIT #−−−−−−−−−−−−−−− # Shell Prompt #−−−−−−−−−−−−−−− if [[ "${DISPLAY#$HOST}" != ":0.0" && "${DISPLAY}" != ":0" ]]; then HILIT=${red} # remote machine: prompt will be partly red else HILIT=${cyan} # local machine: prompt will be partly cyan fi # −−> Replace instances of \W with \w in prompt functions below #+ −−> to get display of full path name. function fastprompt() { unset PROMPT_COMMAND case $TERM in *term | rxvt ) PS1="${HILIT}[\h]$NC \W > \[\033]0;\${TERM} [\u@\h] \w\007\]" ;; linux ) PS1="${HILIT}[\h]$NC \W > " ;; *) PS1="[\h] \W > " ;; esac } function powerprompt() { _powerprompt() { LOAD=$(uptime|sed −e "s/.*: \([^,]*\).*/\1/" −e "s/ //g") } PROMPT_COMMAND=_powerprompt case $TERM in *term | rxvt ) PS1="${HILIT}[\A \$LOAD]$NC\n[\h \#] \W > \[\033]0;\${TERM} [\u@\h] \w\007\]" ;; linux ) PS1="${HILIT}[\A − \$LOAD]$NC\n[\h \#] \w > " ;; * ) PS1="[\A − \$LOAD]\n[\h \#] \w > " ;; esac }
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Advanced Bash−Scripting Guide powerprompt
# this is the default prompt − might be slow # If too slow, use fastprompt instead....
#=============================================================== # # ALIASES AND FUNCTIONS # # Arguably, some functions defined here are quite big # (ie 'lowercase') but my workstation has 512Meg of RAM, so ..... # If you want to make this file smaller, these functions can # be converted into scripts. # # Many functions were taken (almost) straight from the bash−2.04 # examples. # #=============================================================== #−−−−−−−−−−−−−−−−−−− # Personnal Aliases #−−−−−−−−−−−−−−−−−−− alias rm='rm −i' alias cp='cp −i' alias mv='mv −i' # −> Prevents accidentally clobbering files. alias mkdir='mkdir −p' alias alias alias alias alias alias alias alias alias alias alias
h='history' j='jobs −l' r='rlogin' which='type −all' ..='cd ..' path='echo −e ${PATH//:/\\n}' print='/usr/bin/lp −o nobanner −d $LPDEST' pjet='enscript −h −G −fCourier9 −d $LPDEST' background='xv −root −quit −max −rmode 5' du='du −kh' df='df −kTh'
# The alias alias alias alias alias alias alias alias alias alias
'ls' family (this assumes la='ls −Al' ls='ls −hF −−color' lx='ls −lXB' lk='ls −lSr' lc='ls −lcr' lu='ls −lur' lr='ls −lR' lt='ls −ltr' lm='ls −al |more' tree='tree −Csu'
# Assumes LPDEST is defined # Pretty−print using enscript # Put a picture in the background
you use the GNU ls) # show hidden files # add colors for filetype recognition # sort by extension # sort by size # sort by change time # sort by access time # recursive ls # sort by date # pipe through 'more' # nice alternative to 'ls'
# tailoring 'less' alias more='less' export PAGER=less export LESSCHARSET='latin1' export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&−' # Use this if lesspipe.sh exists export LESS='−i −N −w −z−4 −g −e −M −X −F −R −P%t?f%f \ :stdin .?pb%pb\%:?lbLine %lb:?bbByte %bb:−...' # spelling typos − highly personnal :−) alias xs='cd'
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Advanced Bash−Scripting Guide alias alias alias alias
vf='cd' moer='more' moew='more' kk='ll'
#−−−−−−−−−−−−−−−− # a few fun ones #−−−−−−−−−−−−−−−− function xtitle () { case "$TERM" in *term | rxvt) echo −n −e "\033]0;$*\007" ;; *) ;; esac } # aliases... alias top='xtitle Processes on $HOST && top' alias make='xtitle Making $(basename $PWD) ; make' alias ncftp="xtitle ncFTP ; ncftp" # .. and functions function man () { for i ; do xtitle The $(basename $1|tr −d .[:digit:]) manual command man −F −a "$i" done } function ll(){ ls −l "$@"| egrep "^d" ; ls −lXB "$@" 2>&−| egrep −v "^d|total "; } function te() # wrapper around xemacs/gnuserv { if [ "$(gnuclient −batch −eval t 2>&−)" == "t" ]; then gnuclient −q "$@"; else ( xemacs "$@" &); fi } #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # File & strings related functions: #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Find a file with a pattern in name: function ff() { find . −type f −iname '*'$*'*' −ls ; } # Find a file with pattern $1 in name and Execute $2 on it: function fe() { find . −type f −iname '*'$1'*' −exec "${2:−file}" {} \; # find pattern in a set of filesand highlight them: function fstr() { OPTIND=1 local case="" local usage="fstr: find string in files. Usage: fstr [−i] \"pattern\" [\"filename pattern\"] " while getopts :it opt do case "$opt" in i) case="−i " ;;
Appendix J. A Sample .bashrc File
; }
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Advanced Bash−Scripting Guide *) echo "$usage"; return;; esac done shift $(( $OPTIND − 1 )) if [ "$#" −lt 1 ]; then echo "$usage" return; fi local SMSO=$(tput smso) local RMSO=$(tput rmso) find . −type f −name "${2:−*}" −print0 | xargs −0 grep −sn ${case} "$1" 2>&− | \ sed "s/$1/${SMSO}\0${RMSO}/gI" | more } function cuttail() # cut last n lines in file, 10 by default { nlines=${2:−10} sed −n −e :a −e "1,${nlines}!{P;N;D;};N;ba" $1 } function lowercase() # move filenames to lowercase { for file ; do filename=${file##*/} case "$filename" in */*) dirname==${file%/*} ;; *) dirname=.;; esac nf=$(echo $filename | tr A−Z a−z) newname="${dirname}/${nf}" if [ "$nf" != "$filename" ]; then mv "$file" "$newname" echo "lowercase: $file −−> $newname" else echo "lowercase: $file not changed." fi done } function swap() # swap 2 filenames around { local TMPFILE=tmp.$$ mv "$1" $TMPFILE mv "$2" "$1" mv $TMPFILE "$2" }
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # Process/system related functions: #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− function my_ps() { ps $@ −u $USER −o pid,%cpu,%mem,bsdtime,command ; } function pp() { my_ps f | awk '!/awk/ && $0~var' var=${1:−".*"} ; } # This function is roughly the same as 'killall' on linux # but has no equivalent (that I know of) on Solaris function killps() # kill by process name { local pid pname sig="−TERM" # default signal if [ "$#" −lt 1 ] || [ "$#" −gt 2 ]; then echo "Usage: killps [−SIGNAL] pattern"
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Advanced Bash−Scripting Guide return; fi if [ $# = 2 ]; then sig=$1 ; fi for pid in $(my_ps| awk '!/awk/ && $0~pat { print $1 }' pat=${!#} ) ; do pname=$(my_ps | awk '$1~var { print $5 }' var=$pid ) if ask "Kill process $pid with signal $sig?" then kill $sig $pid fi done } function my_ip() # get IP adresses { MY_IP=$(/sbin/ifconfig ppp0 | awk '/inet/ { print $2 } ' | sed −e s/addr://) MY_ISP=$(/sbin/ifconfig ppp0 | awk '/P−t−P/ { print $3 } ' | sed −e s/P−t−P://) } function ii() # get current host related info { echo −e "\nYou are logged on ${RED}$HOST" echo −e "\nAdditionnal information:$NC " ; uname −a echo −e "\n${RED}Users logged on:$NC " ; w −h echo −e "\n${RED}Current date :$NC " ; date echo −e "\n${RED}Machine stats :$NC " ; uptime echo −e "\n${RED}Memory stats :$NC " ; free my_ip 2>&− ; echo −e "\n${RED}Local IP Address :$NC" ; echo ${MY_IP:−"Not connected"} echo −e "\n${RED}ISP Address :$NC" ; echo ${MY_ISP:−"Not connected"} echo } # Misc utilities: function repeat() # repeat n times command { local i max max=$1; shift; for ((i=1; i C−like syntax eval "$@"; done } function ask() { echo −n "$@" '[y/n] ' ; read ans case "$ans" in y*|Y*) return 0 ;; *) return 1 ;; esac } #========================================================================= # # PROGRAMMABLE COMPLETION − ONLY SINCE BASH−2.04 # Most are taken from the bash 2.05 documentation and from Ian McDonalds # 'Bash completion' package (http://www.caliban.org/bash/index.shtml#completion) # You will in fact need bash−2.05a for some features # #========================================================================= if [ "${BASH_VERSION%.*}" \< "2.05" ]; then echo "You will need to upgrade to version 2.05 for programmable completion"
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Advanced Bash−Scripting Guide return fi shopt −s extglob set +o nounset
# necessary # otherwise some completions will fail
complete complete complete complete complete complete complete
−A −A −A −A −A −A −A
hostname export variable enabled alias function user
complete complete complete complete
−A −A −A −A
helptopic help # currently same as builtins shopt shopt stopped −P '%' bg job −P '%' fg jobs disown
complete −A directory complete −A directory
rsh rcp telnet rlogin r ftp ping disk printenv export local readonly unset builtin alias unalias function su mail finger
mkdir rmdir −o default cd
# Compression complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X # Postscript,pdf,dvi..... complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X complete −f −o default −X # Multimedia complete −f −o default −X complete −f −o default −X complete −f −o default −X
'*.+(zip|ZIP)' '!*.+(zip|ZIP)' '*.+(z|Z)' '!*.+(z|Z)' '*.+(gz|GZ)' '!*.+(gz|GZ)' '*.+(bz2|BZ2)' '!*.+(bz2|BZ2)'
zip unzip compress uncompress gzip gunzip bzip2 bunzip2
'!*.ps' gs ghostview ps2pdf ps2ascii '!*.dvi' dvips dvipdf xdvi dviselect dvitype '!*.pdf' acroread pdf2ps '!*.+(pdf|ps)' gv '!*.texi*' makeinfo texi2dvi texi2html texi2pdf '!*.tex' tex latex slitex '!*.lyx' lyx '!*.+(htm*|HTM*)' lynx html2ps '!*.+(jp*g|gif|xpm|png|bmp)' xv gimp '!*.+(mp3|MP3)' mpg123 mpg321 '!*.+(ogg|OGG)' ogg123
complete −f −o default −X '!*.pl'
perl perl5
# This is a 'universal' completion function − it works when commands have # a so−called 'long options' mode , ie: 'ls −−all' instead of 'ls −a' _get_longopts () { $1 −−help | sed −e '/−−/!d' −e 's/.*−−\([^[:space:].,]*\).*/−−\1/'| \ grep ^"$2" |sort −u ; } _longopts_func () { case "${2:−*}" in −*) ;;
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Advanced Bash−Scripting Guide *) esac
return ;;
case "$1" in \~*) eval cmd="$1" ;; *) cmd="$1" ;; esac COMPREPLY=( $(_get_longopts ${1} ${2} ) ) } complete complete
−o default −F _longopts_func configure bash −o default −F _longopts_func wget id info a2ps ls recode
_make_targets () { local mdef makef gcmd cur prev i COMPREPLY=() cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD−1]} # if prev argument is −f, return possible filename completions. # we could be a little smarter here and return matches against # `makefile Makefile *.mk', whatever exists case "$prev" in −*f) COMPREPLY=( $(compgen −f $cur ) ); return 0;; esac # if we want an option, return the possible posix options case "$cur" in −) COMPREPLY=(−e −f −i −k −n −p −q −r −S −s −t); return 0;; esac # make reads `makefile' before `Makefile' if [ −f makefile ]; then mdef=makefile elif [ −f Makefile ]; then mdef=Makefile else mdef=*.mk # local convention fi # before we scan for targets, see if a makefile name was specified # with −f for (( i=0; i < ${#COMP_WORDS[@]}; i++ )); do if [[ ${COMP_WORDS[i]} == −*f ]]; then eval makef=${COMP_WORDS[i+1]} # eval for tilde expansion break fi done [ −z "$makef" ] && makef=$mdef # if we have a partial word to complete, restrict completions to # matches of that word if [ −n "$2" ]; then gcmd='grep "^$2"' ; else gcmd=cat ; fi # if we don't want to use *.mk, we can take out the cat and use # test −f $makef and input redirection COMPREPLY=( $(cat $makef 2>/dev/null | awk 'BEGIN {FS=":"} /^[^.#
][^=]*:/ {print $1}' | tr
}
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Advanced Bash−Scripting Guide complete −F _make_targets −X '+($*|*.[cho])' make gmake pmake
# cvs(1) completion _cvs () { local cur prev COMPREPLY=() cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD−1]} if [ $COMP_CWORD −eq 1 ] || [ COMPREPLY=( $( compgen −W export history import log tag update' $cur )) else COMPREPLY=( $( compgen −f fi return 0
"${prev:0:1}" = "−" ]; then 'add admin checkout commit diff \ rdiff release remove rtag status \
$cur ))
} complete −F _cvs cvs _killall () { local cur prev COMPREPLY=() cur=${COMP_WORDS[COMP_CWORD]} # get a list of processes (the first sed evaluation # takes care of swapped out processes, the second # takes care of getting the basename of the process) COMPREPLY=( $( /usr/bin/ps −u $USER −o comm | \ sed −e '1,1d' −e 's#[]\[]##g' −e 's#^.*/##'| \ awk '{if ($0 ~ /^'$cur'/) print $0}' )) return 0 } complete −F _killall killall killps
# # # #
A meta−command completion function for commands like sudo(8), which need to first complete on a command, then complete according to that command's own completion definition − currently not quite foolproof (e.g. mount and umount don't work properly), but still quite useful − By Ian McDonald, modified by me.
_my_command() { local cur func cline cspec COMPREPLY=() cur=${COMP_WORDS[COMP_CWORD]} if [ $COMP_CWORD = 1 ]; then COMPREPLY=( $( compgen −c $cur ) ) elif complete −p ${COMP_WORDS[1]} &>/dev/null; then cspec=$( complete −p ${COMP_WORDS[1]} ) if [ "${cspec%%−F *}" != "${cspec}" ]; then # complete −F # # COMP_CWORD and COMP_WORDS() are not read−only, # so we can set them before handing off to regular
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Advanced Bash−Scripting Guide # completion routine # set current token number to 1 less than now COMP_CWORD=$(( $COMP_CWORD − 1 )) # get function name func=${cspec#*−F } func=${func%% *} # get current command line minus initial command cline="${COMP_LINE#$1 }" # split current command line tokens into array COMP_WORDS=( $cline ) $func $cline elif [ "${cspec#*−[abcdefgjkvu]}" != "" ]; then # complete −[abcdefgjkvu] #func=$( echo $cspec | sed −e 's/^.*\(−[abcdefgjkvu]\).*$/\1/' ) func=$( echo $cspec | sed −e 's/^complete//' −e 's/[^ ]*$//' ) COMPREPLY=( $( eval compgen $func $cur ) ) elif [ "${cspec#*−A}" != "$cspec" ]; then # complete −A func=${cspec#*−A } func=${func%% *} COMPREPLY=( $( compgen −A $func $cur ) ) fi else COMPREPLY=( $( compgen −f $cur ) ) fi }
complete −o default −F _my_command nohup exec eval trace truss strace sotruss gdb complete −o default −F _my_command command type which man nice # # # #
Local Variables: mode:shell−script sh−shell:bash End:
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Appendix K. Converting DOS Batch Files to Shell Scripts Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file language allowed writing some fairly powerful scripts and applications, though they often required extensive kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a Unix shell script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent shell scripting ones.
Table K−1. Batch file keywords / variables / operators, and their shell equivalents Batch File Operator Shell Script Equivalent % $ / − \ / == = !==! != | | @ set +v * * > > >> >> < < %VAR% $VAR REM # NOT ! NUL /dev/null ECHO echo ECHO. echo ECHO OFF set +v FOR %%VAR IN (LIST) DO for var in [list]; do :LABEL none (unnecessary) GOTO none (use a function) PAUSE sleep CHOICE case or select IF if IF EXIST FILENAME if [ −e filename ] IF !%N==! if [ −z "$N" ] CALL source or . (dot operator) COMMAND /C source or . (dot operator) SET export
Meaning command−line parameter prefix command option flag directory path separator (equal−to) string comparison test (not equal−to) string comparison test pipe do not echo current command filename "wild card" file redirection (overwrite) file redirection (append) redirect stdin environmental variable comment negate following test "black hole" for burying command output echo (many more option in Bash) echo blank line do not echo command(s) following "for" loop label jump to another location in the script pause or wait an interval menu choice if−test test if file exists if replaceable parameter "N" not present "include" another script "include" another script (same as CALL) set an environmental variable
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Advanced Bash−Scripting Guide SHIFT SGN ERRORLEVEL CON PRN LPT1 COM1
shift −lt or −gt $? stdin /dev/lp0 /dev/lp0 /dev/ttyS0
left shift command−line argument list sign (of integer) exit status "console" (stdin) (generic) printer device first printer device first serial port
Batch files usually contain DOS commands. These must be translated into their Unix equivalents in order to convert a batch file into a shell script.
Table K−2. DOS commands and their Unix equivalents DOS Command ASSIGN ATTRIB CD CHDIR CLS COMP COPY Ctl−C Ctl−Z DEL DELTREE DIR ERASE EXIT FC FIND MD MKDIR MORE MOVE PATH REN RENAME RD RMDIR SORT TIME TYPE
Unix Equivalent ln chmod cd cd clear diff, comm, cmp cp Ctl−C Ctl−D rm rm −rf ls −l rm exit comm, cmp grep mkdir mkdir more mv $PATH mv mv rmdir rmdir sort date cat
Effect link file or directory change file permissions change directory change directory clear screen file compare file copy break (signal) EOF (end−of−file) delete file(s) delete directory recursively directory listing delete file(s) exit current process file compare find strings in files make directory make directory text file paging filter move path to executables rename (move) rename (move) remove directory remove directory sort file display system time output file to stdout
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Advanced Bash−Scripting Guide XCOPY
cp
(extended) file copy
Virtually all Unix and shell operators and commands have many more options and enhancements than their DOS and batch file equivalents. Many DOS batch files rely on auxiliary utilities, such as ask.com, a crippled counterpart to read. DOS supports a very limited and incompatible subset of filename wildcard expansion, recognizing only the * and ? characters. Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better than the original.
Example K−1. VIEWDATA.BAT: DOS Batch File REM VIEWDATA REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS" REM BY PAUL SOMERSON
@ECHO OFF IF !%1==! GOTO VIEWDATA REM IF NO COMMAND−LINE ARG... FIND "%1" C:\BOZO\BOOKLIST.TXT GOTO EXIT0 REM PRINT LINE WITH STRING MATCH, THEN EXIT. :VIEWDATA TYPE C:\BOZO\BOOKLIST.TXT | MORE REM SHOW ENTIRE FILE, 1 PAGE AT A TIME. :EXIT0
The script conversion is somewhat of an improvement.
Example K−2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT #!/bin/bash # Conversion of VIEWDATA.BAT to shell script. DATAFILE=/home/bozo/datafiles/book−collection.data ARGNO=1 # @ECHO OFF
Command unnecessary here.
if [ $# −lt "$ARGNO" ] then less $DATAFILE else grep "$1" $DATAFILE fi
# IF !%1==! GOTO VIEWDATA
exit 0
# :EXIT0
# TYPE C:\MYDIR\BOOKLIST.TXT | MORE # FIND "%1" C:\MYDIR\BOOKLIST.TXT
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Advanced Bash−Scripting Guide # GOTOs, labels, smoke−and−mirrors, and flimflam unnecessary. # The converted script is short, sweet, and clean, # which is more than can be said for the original.
Ted Davis' Shell Scripts on the PC site has a set of comprehensive tutorials on the old−fashioned art of batch file programming. Certain of his ingenious techniques could conceivably have relevance for shell scripts.
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Appendix L. Exercises L.1. Analyzing Scripts Examine the following script. Run it, then explain what it does. Annotate the script and rewrite it in a more compact and elegant manner. #!/bin/bash MAX=10000
for((nr=1; nr "... _._. ._. .. .__. _". Hex Dump Do a hex(adecimal) dump on a binary file specified as an argument. The output should be in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4−byte hex number, and the final field the ASCII equivalent of the previous 8 fields. Emulating a Shift Register Using Example 26−14 as an inspiration, write a script that emulates a 64−bit shift register as an array. Implement functions to load the register, shift left, and shift right. Finally, write a function that interprets the register contents as eight 8−bit ASCII characters. Determinant Solve a 4 x 4 determinant. Hidden Words Write a "word−find" puzzle generator, a script that hides 10 input words in a 10 x 10 matrix of random letters. The words may be hidden across, down, or diagonally. Optional: Write a script that solves word−find puzzles. To keep this from becoming too difficult, the solution script will find only horizontal and vertical words. (Hint: Treat each row and column as a string, and search for substrings.) Anagramming Anagram 4−letter input. For example, the anagrams of word are: do or rod row word. You may use /usr/share/dict/linux.words as the reference list. "Word Ladders" A "word ladder" is a sequence of words, with each successive word in the sequence differing from the previous one by a single letter. For example, to "ladder" from mark to vase: mark −−> park −−> part −−> past −−> vast −−> vase
Write a script that solves "word ladder" puzzles. Given a starting and an ending word, the script will list all intermediate steps in the "ladder". Note that all words in the sequence must be "legal." Fog Index
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Advanced Bash−Scripting Guide The "fog index" of a passage of text estimates its reading difficulty, as a number corresponding roughly to a school grade level. For example, a passage with a fog index of 12 should be comprehensible to anyone with 12 years of schooling. The Gunning version of the fog index uses the following algorithm. 1. Choose a section of the text at least 100 words in length. 2. Count the number of sentences (a portion of a sentence truncated by the boundary of the text section counts as one). 3. Find the average number of words per sentence. AVE_WDS_SEN = TOTAL_WORDS / SENTENCES 4. Count the number of "difficult" words in the segment −− those containing at least 3 syllables. Divide this quantity by total words to get the proportion of difficult words. PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS 5. The Gunning fog index is the sum of the above two quantities, multiplied by 0.4, then rounded to the nearest integer. G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) ) Step 4 is by far the most difficult portion of the exercise. There exist various algorithms for estimating the syllable count of a word. A rule−of−thumb formula might consider the number of letters in a word and the vowel−consonant mix. A strict interpretation of the Gunning Fog index does not count compound words and proper nouns as "difficult" words, but this would enormously complicate the script. Calculating PI using Buffon's Needle The Eighteenth Century French mathematician de Buffon came up with a novel experiment. Repeatedly drop a needle of length "n" onto a wooden floor composed of long and narrow parallel boards. The cracks separating the equal−width floorboards are a fixed distance "d" apart. Keep track of the total drops and the number of times the needle intersects a crack on the floor. The ratio of these two quantities turns out to be a fractional multiple of PI. In the spirit of Example 12−39, write a script that runs a Monte Carlo simulation of Buffon's Needle. To simplify matters, set the needle length equal to the distance between the cracks, n = d. Hint: there are actually two critical variables: the distance from the center of the needle to the crack nearest to it, and the angle of the needle to that crack. You may use bc to handle the calculations. Playfair Cipher Implement the Playfair (Wheatstone) Cipher in a script. The Playfair Cipher encrypts text by substitution of "digrams" (2−letter groupings). It is traditional to use a 5 x 5 letter scrambled−alphabet key square for the encryption and decryption. C A I P V
O B K Q W
D F L R X
E G M T Y
S H N U Z
Each letter of the alphabet appears once, except "I" also represents "J". The arbitrarily chosen key word, "CODES" comes first, then all
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Advanced Bash−Scripting Guide the rest of the alphabet, in order from left to right, skipping letters already used. To encrypt, separate the plaintext message into digrams (2−letter groups). If a group has two identical letters, delete the second, and form a new group. If there is a single letter left over at the end, insert a "null" character, typically an "X". THIS IS A TOP SECRET MESSAGE TH IS IS AT OP SE CR ET ME SA GE For each digram, there are three possibilities. −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 1) Both letters will be on the same row of the key square For each letter, substitute the one immediately to the right, in that row. If necessary, wrap around left to the beginning of the row. or 2) Both letters will be in the same column of the key square For each letter, substitute the one immediately below it, in that row. If necessary, wrap around to the top of the column. or 3) Both letters will form the corners of a rectangle within the key square. For each letter, substitute the one on the other corner the rectangle which lies on the same row.
The "TH" digram falls under case #3. G H M N T U (Rectangle with "T" and "H" at corners) T −−> U H −−> G
The "SE" digram falls under case #1. C O D E S (Row containing "S" and "E") S −−> C E −−> S
(wraps around left to beginning of row)
========================================================================= To decrypt encrypted text, reverse the above procedure under cases #1 and #2 (move in opposite direction for substitution). Under case #3, just take the remaining two corners of the rectangle.
Helen Fouche Gaines' classic work, "Elementary Cryptanalysis" (1939), gives a fairly detailed rundown on the Playfair Cipher and its solution methods.
This script will have three main sections I. Generating the "key square", based on a user−input keyword. II. Encrypting a "plaintext" message. Appendix L. Exercises
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Advanced Bash−Scripting Guide III. Decrypting encrypted text. The script will make extensive use of arrays and functions. −− Please do not send the author your solutions to these exercises. There are better ways to impress him with your cleverness, such as submitting bugfixes and suggestions for improving this book.
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Appendix M. Revision History This document first appeared as a HOWTO in the late spring of 2000. Since then, it has gone through many updates and revisions. This book could not have been written without the assistance of the Linux community, and especially of the Linux Documentation Project.
Table M−1. Revision History Release 0.1 0.2 0.3 0.4 0.5 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
Date 14 Jun 2000 30 Oct 2000 12 Feb 2001 08 Jul 2001 03 Sep 2001 14 Oct 2001 06 Jan 2002 31 Mar 2002 02 Jun 2002 16 Jun 2002 13 Jul 2002 29 Sep 2002 05 Jan 2003 10 May 2003 21 Jun 2003 24 Aug 2003 14 Sep 2003 31 Oct 2003 03 Jan 2004 25 Jan 2004 15 Feb 2004 15 Mar 2004 18 Apr 2004
Comments Initial release. Bugs fixed, plus much additional material and more example scripts. Major update. Complete revision and expansion of the book. Major update: Bugfixes, material added, sections reorganized. Stable release: Bugfixes, reorganization, material added. Bugfixes, material and scripts added. Bugfixes, material and scripts added. TANGERINE release: A few bugfixes, much more material and scripts added. MANGO release: A number of typos fixed, more material and scripts. PAPAYA release: A few bugfixes, much more material and scripts added. POMEGRANATE release: Bugfixes, more material, one more script. COCONUT release: A couple of bugfixes, more material, one more script. BREADFRUIT release: A number of bugfixes, more scripts and material. PERSIMMON release: Bugfixes, and more material. GOOSEBERRY release: Major update. HUCKLEBERRY release: Bugfixes, and more material. CRANBERRY release: Major update. STRAWBERRY release: Bugfixes and more material. MUSKMELON release: Bugfixes. STARFRUIT release: Bugfixes and more material. SALAL release: Minor update. MULBERRY release: Minor update.
Appendix M. Revision History
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Appendix N. To Do List • A comprehensive survey of incompatibilities between Bash and the classic Bourne shell. • Same as above, but for the Korn shell (ksh). • A primer on CGI programming, using Bash. Any volunteers?
Appendix N. To Do List
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Appendix O. Copyright The "Advanced Bash Scripting Guide" is copyright © 2000, by Mendel Cooper. The author also asserts copyright on all previous versions of this document. This blanket copyright recognizes and protects the rights of the contributors to this document. This document may only be distributed subject to the terms and conditions set forth in the Open Publication License (version 1.0 or later), http://www.opencontent.org/openpub/. The following license options also apply. A.
Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.
B.
Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.
Provision A, above, explicitly prohibits relabeling this document. An example of relabeling is the insertion of company logos or navigation bars into the cover, title page, or the text. The author grants the following exemptions. 1. Non−profit organizations, such as the Linux Documentation Project and Sunsite. 2. "Pure−play" Linux distributors, such as Debian, Red Hat, Mandrake, SuSE, and others. Without explicit written permission from the author, distributors and publishers (including on−line publishers) are prohibited from imposing any additional conditions, strictures, or provisions on this document or any previous version of it. As of this update, the author asserts that he has not entered into any contractual obligations that would alter the foregoing declarations. Essentially, you may freely distribute this book in unaltered electronic form. You must obtain the author's permission to distribute a substantially modified version or derivative work. The purpose of this restriction is to preserve the artistic integrity of this document and to prevent "forking." If you display or distribute this document or any previous version thereof under any license except the one above, then you are required to obtain the author's written permission. Failure to do so may terminate your distribution rights. These are very liberal terms, and they should not hinder any legitimate distribution or use of this book. The author especially encourages the use of this book for classroom and instructional purposes. The commercial print and other rights to this book are available. Please contact the author if interested. The author produced this book in a manner consistent with the spirit of the LDP Manifesto.
Linux is a trademark registered to Linus Torvalds. Unix and UNIX are trademarks registered to the Open Group.
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Advanced Bash−Scripting Guide MS Windows is a trademark registered to the Microsoft Corp. Pentium is a trademark registered to Intel, Inc. Scrabble is a trademark registered to Hasbro, Inc. All other commercial trademarks mentioned in the body of this work are registered to their respective owners. Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French, German, Italian, Russian, and Chinese translations are also available or in progress. If you wish to translate this document into another language, please feel free to do so, subject to the terms stated above. The author wishes to be notified of such efforts. Notes [1] [2] [3]
[4] [5]
These are referred to as builtins, features internal to the shell. Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash. By convention, user−written shell scripts that are Bourne shell compliant generally take a name with a .sh extension. System scripts, such as those found in /etc/rc.d, do not conform to this nomenclature. Some flavors of Unix (those based on 4.2BSD) take a four−byte magic number, requiring a blank after the ! −− #! /bin/sh. The #! line in a shell script will be the first thing the command interpreter (sh or bash) sees. Since this line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally executes the script. The line has already served its purpose − calling the command interpreter. If, in fact, the script includes an extra #! line, then bash will interpret it as a comment. #!/bin/bash echo "Part 1 of script." a=1 #!/bin/bash # This does *not* launch a new script. echo "Part 2 of script." echo $a # Value of $a stays at 1.
[6]
This allows some cute tricks. #!/bin/rm # Self−deleting script. # Nothing much seems to happen when you run this... except that the file disappears. WHATEVER=65 echo "This line will never print (betcha!)." exit $WHATEVER
Appendix O. Copyright
# Doesn't matter. The script will not exit here.
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Advanced Bash−Scripting Guide Also, try starting a README file with a #!/bin/more, and making it executable. The result is a self−listing documentation file. [7] Portable Operating System Interface, an attempt to standardize UNIX−like OSes. The POSIX specifications are listed on the Open Group site. [8] Caution: invoking a Bash script by sh scriptname turns off Bash−specific extensions, and the script may therefore fail to execute. [9] A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it. [10] Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where scriptname is located, why doesn't this work? This fails because, for security reasons, the current directory, "." is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a ./scriptname. [11] The shell does the brace expansion. The command itself acts upon the result of the expansion. [12] Exception: a code block in braces as part of a pipe may be run as a subshell. ls | { read firstline; read secondline; } # Error. The code block in braces runs as a subshell, # so the output of "ls" cannot be passed to variables within the block. echo "First line is $firstline; second line is $secondline" # Will not work. # Thanks, S.C.
[13] The process calling the script sets the $0 parameter. By convention, this parameter is the name of the script. See the manpage for execv. [14] Encapsulating "!" within double quotes gives an error when used from the command line. This is interpreted as a history command. Within a script, though, this problem does not occur, since the Bash history mechanism is disabled then. Of more concern is the inconsistent behavior of "\" within double quotes. bash$ echo hello\! hello!
bash$ echo "hello\!" hello\!
bash$ echo −e x\ty xty
bash$ echo −e "x\ty" x y
(Thank you, Wayne Pollock, for pointing this out.) [15] "Word splitting", in this context, means dividing a character string into a number of separate and discrete arguments. [16] Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts. [17] On modern Unix systems, the sticky bit is no longer used for files, only on directories. Appendix O. Copyright
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Advanced Bash−Scripting Guide [18] As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ −n "$string" −o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x −o "x$a" = "x$b" ] (the "x's" cancel out). [19] The PID of the currently running script is $$, of course. [20] The words "argument" and "parameter" are often used interchangeably. In the context of this document, they have the same precise meaning, that of a variable passed to a script or function. [21] This applies to either command line arguments or parameters passed to a function. [22] If $parameter is null in a non−interactive script, it will terminate with a 127 exit status (the Bash error code for "command not found"). [23] These are shell builtins, whereas other loop commands, such as while and case, are keywords. [24] An exception to this is the time command, listed in the official Bash documentation as a keyword. [25] A option is an argument that acts as a flag, switching script behaviors on or off. The argument associated with a particular option indicates the behavior that the option (flag) switches on or off. [26] The C source for a number of loadable builtins is typically found in the /usr/share/doc/bash−?.??/functions directory.
[27] [28]
[29] [30] [31]
Note that the −f option to enable is not portable to all systems. The same effect as autoload can be achieved with typeset −fu. These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show up in a normal ls listing, and they cannot be deleted by an accidental rm −rf *. Dotfiles are generally used as setup and configuration files in a user's home directory. This is only true of the GNU version of tr, not the generic version often found on commercial Unix systems. A tar czvf archive_name.tar.gz * will include dotfiles in directories below the current working directory. This is an undocumented GNU tar "feature". This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed to the "public key" cipher class, of which pgp is a well−known example.
[32] A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events.
[33] [34] [35] [36]
The word "daemon" means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way Unix daemons silently wander about behind the scenes, carrying out their appointed tasks. This is actually a script adapted from the Debian Linux distribution. The print queue is the group of jobs "waiting in line" to be printed. For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the September, 1997 issue of Linux Journal. EBCDIC (pronounced "ebb−sid−ic") is an acronym for Extended Binary Coded Decimal Interchange Code. This is an IBM data format no longer in much use. A bizarre application of the conv=ebcdic option of dd is as a quick 'n easy, but not very secure text file encoder. cat $file | dd conv=swab,ebcdic > $file_encrypted # Encode (looks like gibberish). # Might as well switch bytes (swab), too, for a little extra obscurity.
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Advanced Bash−Scripting Guide cat $file_encrypted | dd conv=swab,ascii > $file_plaintext # Decode.
[37] A macro is a symbolic constant that expands into a command string or a set of operations on parameters. [38] This is the case on a Linux machine or a Unix system with disk quotas. [39] The userdel command will fail if the particular user being deleted is still logged on. [40] For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal. [41] The −c option to mke2fs also invokes a check for bad blocks. [42] Operators of single−user Linux systems generally prefer something simpler for backups, such as tar. [43] NAND is the logical "not−and" operator. Its effect is somewhat similar to subtraction. [44] For purposes of command substitution, a command may be an external system command, an internal scripting builtin, or even a script function. [45] In a more technically correct sense, command substitution extracts the stdout of a command, then assigns it to a variable using the = operator. [46] A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified version of a file pointer. It is analogous to a file handle in C. [47] Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e−mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone. [48] The simplest type of Regular Expression is a character string that retains its literal meaning, not containing any metacharacters. [49] Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those cases where there is a newline in a multiple line expression, the dot will match the newline. #!/bin/bash sed −e 'N;s/.*/[&]/'
